2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

2-STAGE EVAPORATIVE COOLING UNITS A 2-stage evaporative cooling unit basically consists of an indirect cooling stage where air is cooled over a cooling coil with water from a cooling tower. This in the indirect stage. The temperature of the air is the further reduced in an evaporator pad through direct contact between water and air (direct stage). This exercise looks at various configurations with differeing levels of effectiveness and investigates what temperatures can be achieved with evaporative cooling units. Most of the recent interest for evaporative cooling comes from Pretoria and it is therefore that all examples and claculations below are for Pretoria altitude and climate data. "STANDARD" UNIT This is the unit most commonly used for comfort as well as for industrial applications.

The above configuration, with return air fan, return air plenum and mixing plenum is for use in a conventional Central Variable Volume air-conditioning system. The supply- and return air fans are variable volume (speed control), controlled by the supply air riser static pressure. It is the intention to control the supply air temperature as close to constant as possible. Control methods will be discussed later. The unit shown above is the "compact" unit: The cooling tower pack (top) and the direct evaporative cooling pack (bottom) form one continuous unit with a water distribution section at the top and a sump with circulation pump at the bottom. The compact 2-stage evaporative cooling unit consists of the following: 1 The filter bank covering both the coil intake (primary air) and the cooling tower intake (secondary air). 2 The cooling coil: This is the first step of cooling (indirect evaporative cooling). 3 The direct cooling evaporator pack with sump and circulation pump. On top of it, and forming part of it, is the cooling tower pack with the water distribution set. 4 The cooling tower fan (axial). (Secondary air). 5 The supply air fan (Primary air). The return air fan plus the plenums and dampers have been introduced to adapt the 2-stage evaporative cooling unit to comfort air-conditioning in a (large) central system where traditionally an all-air system would have been used, with chilled water as cooling source. Note that there is no heater bank in this unit. This will be explained later when the controls are discussed.

Page 1 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

The "SPLIT" UNIT It is a lot easier to explain the 2-stage evaporative cooling principle with a split system:

OUTSIDE AIR

COOLING TOWER

RETURN AIR RELIEF AIR

The system now consists of and air handling unit and a cooling tower. Those units are separate and the cooling tower can even be mounted in a remote location. (As long as the sump of the cooling tower is higher than the top of the cooling coil of the air handling unit.) The humidifier is now a stand-alone unit with its own pack, sump and water distribution system. Under Pretoria design conditions, and for a nominal capacity of 10 m 3/s:

Page 2 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

Psychrometrics and Heat Balances: DB WB (oC) (oC) 3 Cooling coil 10.0 m /s 30.0 17.9 22.0 15.0 20.0 13.6 30.0 20.5 Air in: 32.8 19.6 Air out: 23.9 16.9 Δenthalpy: 3 Cooling: 10.0 m /s x 3 Cooling Tower 10.0 m /s Air in: 32.8 19.6 Air out: 21.9 21.7 Δenthalpy: 3 Heat rejected 10.0 m /s x Water:

5.0 kg/s @

RH (%)

AH (kg/kg)

DP (oC)

31 49 50 43 29 50

0.0100 0.0096 0.0087 0.0138 0.0113 0.0113

11.5 10.9 9.5 16.4 13.3 13.3

3 1.01 kg/m x

29 98

0.0113 0.0193

3 1.01 kg/m x

in: out:

Enth (kJ/kg)

Sp.M (kg/m3)

55.9 46.8 42.4 65.7 62.1 53.0 9.2 9.2 kJ/kg =

0.97 1.00 1.01 0.97 0.96 0.99

13.3 21.6

62.1 71.3 9.2 9.2 kJ/kg =

o 25.8 C o 21.4 C -4.4 degC x

= Nett Room Sensible Cooling Capacity (NRSCC),

with fan heat pick-up: and room design temp:

92.5 kW 0.96 0.98 92.1 kW

4.19 kJ/kg-degC -92.3 kW 1.0 degC o 23.5 C

Unit leaving Room entering Room condition(*) NRSCC

17.6 16.8 93 0.0139 16.4 53.0 1.01 18.6 17.1 87 0.0139 16.4 54.0 1.00 23.5 18.6 64 0.0139 16.4 59.1 0.99 3 3 10.0 m /s x 1.01 kg/m x (1.012 + 1.89*0.0140) x (23.5-18.6) degC = 51.1 kW 3 NRSCC/Primary Air: 5.1 kW per m /s 3 NRSCC/Total Air: 2.6 kW per m /s (*)

Room latent heat gain ignored.

The Psychometrics and heat balances for the COMPACT UNIT are rather more complicated:

Page 3 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

Psychrometrics and Heat Balances DB WB (oC) (oC) 3 Cooling coil 10.0 m /s Air in: 32.8 19.6 Air out: 21.6 16.2 Δenthalpy: 3 Cooling: 10.0 m /s x 3 Humidifier pack 10.0 m /s Air in: 21.6 16.2 Air out: 17.9 17.3 Δenthalpy: 3 Reheat in Pack: 10.0 m /s x 3 Cooling tower 10.0 m /s Air in: 32.8 19.6 Air out: 21.5 21.2 Δenthalpy: 3 Heat rejection: 10.0 m /s x Water - Cooling tower:

Water - Humidifier pack:

Water - Cooling coil:

3.0 kg/s @

3.0 kg/s @

3.0 kg/s @

Nett Room Sensible Cooling Capacity (NRSCC),

RH (%)

AH (kg/kg)

DP (oC)

29 58

0.0113 0.0113

13.3 13.3

3 1.01 kg/m x

58 95

0.0113 0.0145

3 1.01 kg/m x

29 98

0.0113 0.0188

3 1.01 kg/m x

in: out:

in: out:

in: out:

Enth (kJ/kg)

Sp.M (kg/m3)

62.1 50.5 11.6 11.6 kJ/kg =

0.96 1.00

13.3 17.1

50.5 54.8 -4.3 -4.3 kJ/kg =

1.00 1.00

13.3 21.1

0.96 0.99

62.1 69.4 -7.3 -7.3 kJ/kg =

o 26.3 C o 20.5 C 5.8 degC x

117 kW

-43 kW

-73 kW

=

4.19 kJ/kg-degC 73 kW

=

4.19 kJ/kg-degC 44 kW

=

4.19 kJ/kg-degC -117 kW

o 20.5 C o 17.0 C 3.5 degC x o 17.0 C o 26.3 C -9.3 degC x

with fan heat pick-up: and room design temp:

1.0 degC o 23.5 C

Unit leaving Room entering Room condition(*) NRSCC

17.9 17.3 95 0.0145 17.1 54.8 1.00 18.9 17.6 89 0.0145 17.1 55.9 1.00 23.5 19.0 67 0.0145 17.1 60.7 0.99 3 3 10.0 m /s x 1.01 kg/m x (1.012 + 1.89*0.0144) x (23.5-18.9) degC = 48.3 kW 3 NRSCC/Primary Air: 4.8 kW per m /s 3 NRSCC/Total Air: 2.4 kW per m /s (*)

Room latent heat gain ignored.

What happens in the humidifier pack (the second or direct stage of evaporative cooling)? There is a sensible cooling from 21.6 to 17.9 o C, but this is not an adiabatic cooling process: The air returns 43 kW of the 117 kW cooling from the cooling coil. The humidifier pack actually acts as a 2 nd stage cooling tower: It keeps on cooling the water by another 3.5 degC. This brings the water temperature down to 17 o C and this increases the cooling coil capacity to 117 kW. But it is only borrowed: The air has to return a portion of it in the humidifer pack.

Page 4 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

Comparison between Split and Compact Unit (With identical heat exchanging media)

1

Psychrometrics

2

The achievable temperature of a split unit is about 0.3 degC better, and its nett cooling capacity 6% higher. On the other hand, the compact unit is simpler with only one pump, sump and water distribution system and therfore less expensive. Why is the water quantity for the split unit 5 l/s (in our example) against only 3 l/s for the compact unit? Trial and error with the compact unit computer model shows that there is an optimum water quantity (in our case around 2.75 l/s) above which the performance starts to drop off again. The split unit is more straight forward: The more water the better. Above 5 l/s however, the improvement in performance is not worth the extra cost. The Compact unit: The intermediate conditions (between the coil and the humidifier pack) are: 21.6 oCdb/16.2 oCwb. Is it worthwhile diverting some of that air to achieve lower supply air conditions? The standard unit (compact or split) with a supply air temperature of around 17.5 to 18.0 oC is more than adequate for most applications. However, there might be situations where a lower supply temperature is required be it at the expense of a lower air quantity. The following examples will explore how low we can go.

3 4

5

Page 5 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

Diverting 50% of the air

50% Of the air is diverted between cooling coil (COIL) and HPACK1 and taken through its own direct evaporative cooling pack (HPACK2). The result is a supply air temperature of 16.9 oC. The rest of the primary air goes through HPACK1 and emerges at 18.2 oC wetbulb The effect on the unit itself (water temperature, cooling coil, …) is small. Psychrometrics and Heat Balances DB o ( C) 3 COIL 10.0 m /s Air in: 32.8 Air out: 22.0 Δenthalpy: Cooling: 10.0 3 HPACK1 5.0 m /s Air in: 22.0 Air out: 19.0 Δenthalpy: Reheat in Pack: 5.0 3 CTPACK 10.0 m /s Air in: 32.8 Air out: 21.3 Δenthalpy: Heat rejection: 10.0 Water - CTPACK:

Water - HPACK1:

WB RH o ( C) (%) (Cooling Coil) 19.6 29 16.3 57

AH (kg/kg)

DP ( C)

0.0113 0.0113

13.3 13.3

3 m3/s x 1.01 kg/m x (Humidifier pack) 16.3 57 0.0113 18.3 94 0.0154 3 m3/s x 1.01 kg/m x (Cooling tower) 19.6 29 0.0113 21.3 100 0.0189

m3/s x

3.0 kg/s @

3.0 kg/s @

3 1.01 kg/m x

in: out:

in: out:

Enth (kJ/kg)

Sp.M (kg/m3)

62.1 51.0 11.2 11.2 kJ/kg =

0.96 1.00

13.3 18.0

51.0 58.3 -7.3 -7.3 kJ/kg =

1.00 1.00

13.3 21.3

0.96 0.99

o

62.1 69.7 7.5 7.5 kJ/kg =

o 26.5 C o 20.5 C 6.0 degC x

-36.6 kW

75.7 kW

=

4.19 kJ/kg-degC 75.7 kW

=

4.19 kJ/kg-degC 36.6 kW

o 20.5 C o 17.6 C 2.9 degC x

Page 6 of 49

112.3 kW

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

Water - COIL

3.0 kg/s @

in: out:

o 17.6 C o 26.5 C -8.9 degC x

= Nett Room Sensible Cooling Capacity (NRSCC), 3 Primary Air 5.0 m /s

4.19 kJ/kg-degC -112.3 kW

with fan heat pick-up: and room design temp:

1.0 degC o 23.5 C

Unit leaving Room entering Room condition(*) NRSCC

16.9 16.2 94 0.0134 15.9 51.0 1.01 17.9 16.5 88 0.0134 15.9 52.0 1.01 23.5 18.3 61 0.0134 15.9 57.9 0.99 5.0 m3/s x 1.01 kg/m3 x (1.012 + 1.89*0.0134) x (23.5-17.9) degC = 29.4 kW 3 NRSCC/Primary Air: 5.9 kW per m /s 3 NRSCC/Total Air: 1.5 kW per m /s (*)

Room latent heat gain ignored.

Heat Balances Useful Cooling:

3 5.0 m /s

Useful Cooling: Heat Rejection:

+

from COIL entering: to HPACK2 leaving: 3 3 5.0 m /s x 1.01 kg/m x

3 10.0 m /s

from ambient: to cooling tower leaving: 3 3 10.0 m /s x 1.01 kg/m x 3 5.0 m /s from ambient: to HPACK1 leaving: 3 3 5.0 m /s x 1.01 kg/m x

62.1 kJ/kg 51.0 kJ/kg 11.2 kJ/kg = 62.1 69.7 7.5 62.1 58.3 -3.9

kJ/kg kJ/kg kJ/kg = kJ/kg kJ/kg kJ/kg =

Heat Rejection: Alternative Heat Balance: HPAC1 can be used somewhere as useful cooling 3 Useful Cooling: 5.0 m /s from COIL entering: 62.1 kJ/kg to HPACK2 leaving: 51.0 kJ/kg 3 3 5.0 m /s x 1.01 kg/m x 11.2 kJ/kg = 3 + 5.0 m /s from COIL entering: 62.1 kJ/kg to HPACK1 leaving: 58.3 kJ/kg 3 3 5.0 m /s x 1.01 kg/m x 3.9 kJ/kg = Useful Cooling: 3 Heat Rejection: 10.0 m /s from ambient: 62.1 kJ/kg to cooling tower leaving: 69.7 kJ/kg 3 3 10.0 m /s x 1.01 kg/m x 7.5 kJ/kg =

56 kW

76 kW

-20 kW 56 kW

56 kW

20 kW 76 kW

76 kW

The cooling capacity of the air leaving HPAC1 would get lost if that air is rejected to the outside. But maybe it can also be usefully applied somewhere in the building where the higher temperature and humidity are not a problem, or we can send the air through the cooling tower instead of using ambient air.

Page 7 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

50% of air diverted, 50% to cooling tower Pretoria - 1369 m above sea level 1.01 kg/cum nominal density 26.5 C 5 cum/s 22.1 Cwb 73.3 kJ/kg

26.5 C

CTPACK

5 cum/s 18.4 Cwb 58.5 kJ/kg

COIL

20.6 C 5 cum/s

3.0 l/s 17.6 C

HPACK2

5 cum/s

Outside Air - 10 cum/s

22.0/16.3 51.0 kJ/kg

5 cum/s

HPACK1

32.8 Cdb/19.6 Cwb/62.1 kJ/kg

17.6 C

50% DIVERTED, 50% TO COOLING TOWER (Unit 3)

16.9/16.2 51.0 kJ/kg

50% Of the air is diverted between cooling coil (COIL) and HPACK1 and taken through its own direct evaporative cooling pack (HPACK2). The rest of the primary air goes through HPACK1 and then through the cooling tower. The total amount of air is halved. Psychrometrics and Heat Balances DB (oC) 3 COIL 10.0 m /s Air in: 32.8 Air out: 22.0 Δenthalpy: Cooling: 10.0 3 HPACK1 5.0 m /s Air in: 22.0 Air out: 19.0 Δenthalpy: Reheat in Pack: 5.0 3 CTPACK 5.0 m /s Air in: 19.0 Air out: 22.2 Δenthalpy: Heat rejection: 5.0

WB RH (oC) (%) (Cooling Coil) 19.6 29 16.3 57

AH (kg/kg)

DP (oC)

0.0113 0.0113

13.3 13.3

3 m3/s x 1.01 kg/m x (Humidifier pack) 16.3 57 0.0113 18.4 95 0.0155 3 m3/s x 1.01 kg/m x (Cooling tower) 18.4 94 0.0155 22.1 99 0.0200

m3/s x

3 1.01 kg/m x

Page 8 of 49

Enth (kJ/kg)

Sp.M (kg/m3)

62.1 51.0 11.2 11.2 kJ/kg =

0.96 1.00

13.3 18.1

51.0 58.5 -7.6 -7.6 kJ/kg =

1.00 1.00

18.1 22.1

1.00 0.98

58.5 73.3 14.8 14.8 kJ/kg =

112.6 kW

-38.1 kW

74.3 kW

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

Water - CTPACK:

3.0 kg/s @

Water - HPACK1:

3.0 kg/s @

Water - COIL

3.0 kg/s @

Nett Room Sensible Cooling Capacity (NRSCC), 3 Primary Air 5.0 m /s

in: out:

in: out:

in: out:

o 26.5 C o 20.6 C 5.9 degC x

=

4.19 kJ/kg-degC 74.3 kW

=

4.19 kJ/kg-degC 38.1 kW

=

4.19 kJ/kg-degC -112.4 kW

o 20.6 C o 17.6 C 3.0 degC x

o 17.6 C o 26.5 C -8.9 degC x

with fan heat pick-up: and room design temp:

1.0 degC o 23.5 C

Unit leaving Room entering Room condition(*) NRSCC

16.9 16.2 94 0.0134 15.9 51.0 1.01 17.9 16.5 88 0.0134 15.9 52.0 1.01 23.5 18.3 61 0.0134 15.9 57.8 0.99 5.0 m3/s x 1.01 kg/m3 x (1.012 + 1.89*0.0134) x (23.5-17.9) degC = 29.5 kW 3 NRSCC/Primary Air: 5.9 kW per m /s 3 NRSCC/Total Air: 2.9 kW per m /s (*)

Room latent heat gain ignored.

Heat Balances Useful Cooling:

3 5.0 m /s

Useful Cooling: Heat Rejection: Heat Rejection:

3 5.0 m /s

from COIL entering: to HPACK2 leaving: 3 3 5.0 m /s x 1.01 kg/m x

62.1 kJ/kg 51.0 kJ/kg 11.2 kJ/kg =

56 kW

from ambient: to cooling tower leaving: 3 3 5.0 m /s x 1.01 kg/m x

62.1 kJ/kg 73.3 kJ/kg 11.1 kJ/kg =

56 kW

Page 9 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

MORE STAGES The basic 2-stage evaporative cooling units (compact and split) deliver air at around 18oC. One can improve on that by diverting a portion of the air after the cooling coil of a compact unit and give it its own direct evaporative cooling pack. (2½ - stage evap cooling?). The achievable temperature is now around 17 oC. This is an improvement of 25% in terms of Nett Room Sensible Cooling Capacity. For further improvements one has to look at a 3-stage evaporative cooling unit. This is best illustrated on the hand of a split unit.

Stage 1: Stage 2: Stage 3:

Cooling Coil (COIL1) + Cooling Tower (CTPACK1). The air leaving COIL1 is split up in 2 equal parts. Cooling Coil (COIL2) + Cooling Tower (CTPACK2). Direct Evaporative Cooling Pack (HPACK)

Psychrometrics and Heat Balances DB (oC) 3 COIL1 10.0 m /s Air in: 32.8 Air out: 23.9 Δenthalpy: Cooling: 10.0 3 CTPACK1 10.0 m /s Air in: 32.8 Air out: 22.2 Δenthalpy: Heat rejection: 10.0 3 COIL2 5.0 m /s Air in: 23.9 Air out: 19.1 Δenthalpy: Cooling: 5.0 3 CTPACK2 5.0 m /s Air in: 23.9 Air out: 18.5 Δenthalpy: Heat rejection: 5.0

WB RH AH (oC) (%) (kg/kg) (Cooling Coil - Stage 1) 19.6 29 0.0113 16.9 50 0.0113 3 m3/s x 1.01 kg/m x (Cooling tower - Stage 1) 19.6 29 0.0113 21.7 96 0.0192 3 m3/s x 1.01 kg/m x (Cooling Coil - Stage 2) 16.9 50 0.0113 15.3 69 0.0113 3 m3/s x 1.01 kg/m x (Cooling tower - Stage 2) 16.9 50 0.0113 18.2 97 0.0155

m3/s x

3 1.01 kg/m x

Page 10 of 49

DP (oC)

Enth (kJ/kg)

Sp.M (kg/m3)

62.1 53.0 9.2 9.2 kJ/kg =

0.96 0.99

13.3 13.3

13.3 21.5

62.1 71.3 9.1 9.1 kJ/kg =

13.3 13.3

53.0 47.9 5.0 5.0 kJ/kg =

13.3 18.1

53.0 58.0 5.0 5.0 kJ/kg =

92.5 kW 0.96 0.98 91.9 kW 0.99 1.01 25.4 kW 0.99 1.00 25.2 kW

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

Water - CTPACK1:

5.0 kg/s @

Water - COIL1

5.0 kg/s @

Water - CTPACK2:

5.0 kg/s @

Water - COIL2

5.0 kg/s @

Nett Room Sensible Cooling Capacity (NRSCC), 3 Primary Air 5.0 m /s

in: out:

in: out:

in: out:

in: out:

o 25.8 C o 21.4 C 4.4 degC x

=

4.19 kJ/kg-degC 92.3 kW

=

4.19 kJ/kg-degC -92.3 kW

=

4.19 kJ/kg-degC 25.3 kW

=

4.19 kJ/kg-degC -25.3 kW

o 21.4 C o 25.8 C -4.4 degC x

o 19.3 C o 18.1 C 1.2 degC x

o 18.1 C o 19.3 C -1.2 degC x

with fan heat pick-up: and room design temp:

1.0 degC o 23.5 C

Unit leaving Room entering Room condition(*) NRSCC

15.7 15.2 96 0.0127 15.0 47.9 1.02 16.7 15.6 90 0.0127 15.0 48.9 1.01 23.5 17.8 58 0.0127 15.0 56.0 0.99 3 3 5.0 m /s x 1.01 kg/m x (1.012 + 1.89*0.0127) x (23.5-16.7) degC = 35.5 kW 3 NRSCC/Primary Air: 7.1 kW per m /s 3 NRSCC/Total Air: 1.8 kW per m /s (*)

Room latent heat gain ignored.

Heat Balances Useful Cooling:

3 5.0 m /s

Useful Cooling: Heat Rejection:

+

from COIL1 entering: to HPACK leaving: 3 3 5.0 m /s x 1.01 kg/m x

3 10.0 m /s

from ambient: to Cooling Tower1 leaving: 3 3 10.0 m /s x 1.01 kg/m x 3 5.0 m /s from ambient: to Cooling tower 2 leaving: 3 3 5.0 m /s x 1.01 kg/m x

Heat Rejection: There is some cooling capacity left in the air from Cooling Tower 2

Page 11 of 49

62.1 kJ/kg 47.9 kJ/kg 14.2 kJ/kg = 62.1 71.3 9.1 62.1 58.0 -4.2

kJ/kg kJ/kg kJ/kg = kJ/kg kJ/kg kJ/kg =

72 kW

92 kW

-21 kW 71 kW

Use it in Cooling Tower 1.

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

Air From Cooling Tower 2 to Cooling Tower 1

The air leaving Cooling Tower 2 is used to cool Cooling Tower 1 and the total amount of air is now halved. Psychrometrics and Heat Balances DB (oC) 3 COIL1 10.0 m /s Air in: 32.8 Air out: 24.3 Δenthalpy: Cooling: 10.0 3 COIL2 5.0 m /s Air in: 24.3 Air out: 19.3 Δenthalpy: Cooling: 5.0 3 CTPACK2 5.0 m /s Air in: 24.3 Air out: 18.7 Δenthalpy: Heat rejection: 5.0 3 CTPACK1 5.0 m /s Air in: 18.7 Air out: 22.8 Δenthalpy: Heat rejection: 5.0

WB RH AH (oC) (%) (kg/kg) (Cooling Coil - Stage 1) 19.6 29 0.0113 17.0 49 0.0113 3 m3/s x 1.01 kg/m x (Cooling Coil - Stage 2) 17.0 49 0.0113 15.4 68 0.0113 3 m3/s x 1.01 kg/m x (Cooling tower - Stage 2) 17.0 49 0.0113 18.4 97 0.0156 3 m3/s x 1.01 kg/m x (Cooling tower - Stage 1) 18.4 97 0.0156 22.8 100 0.0208

m3/s x

3 1.01 kg/m x

Page 12 of 49

DP (oC)

Enth (kJ/kg)

Sp.M (kg/m3)

62.1 53.3 8.8 8.8 kJ/kg =

0.96 0.99

13.3 13.3

13.3 13.3

53.3 48.1 5.2 5.2 kJ/kg =

13.3 18.3

53.3 58.6 5.2 5.2 kJ/kg =

18.3 22.8

58.6 76.1 17.5 17.5 kJ/kg =

88.6 kW 0.99 1.01 26.4 kW 0.99 1.00 26.3 kW 1.00 0.98 88.2 kW

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

Water - CTPACK1:

Water - COIL1

Water - CTPACK2:

Water - COIL2

5.0 kg/s @

5.0 kg/s @

5.0 kg/s @

5.0 kg/s @

Nett Room Sensible Cooling Capacity (NRSCC), 3 Primary Air 5.0 m /s

in: out:

in: out:

in: out:

in: out:

o 26.1 C o 21.9 C 4.2 degC x

=

4.19 kJ/kg-degC 88.3 kW

=

4.19 kJ/kg-degC -88.3 kW

=

4.19 kJ/kg-degC 26.3 kW

=

4.19 kJ/kg-degC -26.3 kW

o 21.9 C o 26.1 C -4.2 degC x

o 19.5 C o 18.2 C 1.3 degC x

o 18.2 C o 19.5 C -1.3 degC x

with fan heat pick-up: and room design temp:

Unit leaving Room entering Room condition(*) NRSCC

1.0 degC o 23.5 C

15.8 15.3 96 0.0127 15.1 48.1 1.01 16.8 15.6 90 0.0127 15.1 49.1 1.01 23.5 18.0 60 0.0130 15.4 56.9 0.99 3 3 5.0 m /s x 1.01 kg/m x (1.012 + 1.89*0.0127) x (23.5-16.8) degC = 35.2 kW 3 NRSCC/Primary Air: 7.0 kW per m /s 3 NRSCC/Total Air: 3.5 kW per m /s (*) With a Room Sensible Heat Gain Factor of 0.9

Page 13 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

Heat Balances Useful Cooling:

3 5.0 m /s

Useful Cooling: Heat Rejection:

3 5.0 m /s

Heat Rejection:

from COIL 1 entering: to HPACK leaving: 3 3 5.0 m /s x 1.01 kg/m x

62.1 kJ/kg 48.1 kJ/kg 14.0 kJ/kg =

70.7 kW

from ambient: to Cooling Tower 1 leaving: 3 3 5.0 m /s x 1.01 kg/m x

62.1 kJ/kg 76.1 kJ/kg 14.0 kJ/kg =

70.2 kW

CONTROL It is the intention to employ the evaporative cooling unit in a conventional, full-fledged air-conditioning system such as the central, all-air variable volume system. This means that the unit must deliver air at a controlled static pressure and a controlled temperature under all possible ambient conditions. Note that the supply air temperature setpoint is constant and completely independant of what happens downstream in the building. The standard evaporative cooling units deliver air at between 17 and 18 oC under design condition. Therefore, let's set the temperature setpoint at 17.5 oC The split unit is easiest to explain:

COOLING TOWER

HPACK

OUTS/A DAMPER

SUPPLY FAN

RET/A DAMPER COIL

RETURN FAN

REL/A DAMPER

Practical Example:

Psychrometrics:

10 m3/s unit running at 70% with the ouside conditions at 20.0 Cdb/14 Cwb m3/s

DB ( C)

WB (oC)

RH (%)

AH (kg/kg)

DP ( C)

Enth (kJ/kg)

Sp.M (kg/m3)

20.0 14.6 12.8

14.0 13.9 12.4

53 93 96

0.0093 0.0115 0.0105

10.3 13.5 12.2

43.8 43.8 39.5

1.01 1.02 1.03

o

Unit on full outside air No cooling equipment on: Spray pump only: + Cooling tower on:

7.00 7.00 7.00

Page 14 of 49

o

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

The cooling tower will be off. The spay pump will be cycling on/off and the supply temperature will vary between 14.6 and 20.0 oC If the outside temperature drops below 17.7 oC, mix return air (around 23 o) and outside air in the correct proportion to arrive at 17.5 oC. Another Example:

10 m3/s unit running at 80% with the ouside conditions at 30.0 Cdb/18.5 Cwb

Psychrometrics:

m3/s

DB (oC)

WB (oC)

RH (%)

AH (kg/kg)

DP (oC)

Enth (kJ/kg)

Sp.M (kg/m3)

Unit on full outside air No cooling equipment on: Spray pump only: + Cooling tower on:

8.00 8.00 8.00

30.0 19.7 16.4

18.5 18.3 15.8

34 88 94

0.0109 0.0151 0.0130

12.7 17.7 15.4

58.2 58.2 49.6

0.97 1.00 1.01

The spray pump will remain on continously. The cooling tower will cycle and the temperature will vary between between 16.4 and 19.7 oC. The same examples for the compact unit:

COOLING TOWER FAN

CTPACK

HPACK

COIL

RET/A DAMPER

OUTS/A DAMPER

SUPPLY AIR FAN

RETURN FAN

REL/A DAMPER

The main difference with the split system is that, when the cooling tower is off, the coil is still active. This means that there is still a lot of activity even when only direct evaporative cooling is required.

Page 15 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

10 m3/s unit running at 70% with the ouside conditions at 20.0 Cdb/14 Cwb

First Example:

Psychrometrics:

m3/s

DB ( C)

WB (oC)

RH (%)

AH (kg/kg)

DP ( C)

Enth (kJ/kg)

Sp.M (kg/m3)

20.0 14.2 13.1

14.0 13.9 12.9

53 97 98

0.0093 0.0116 0.0109

10.3 13.7 12.8

43.8 43.8 40.8

1.01 1.02 1.03

o

Unit on full outside air No cooling equipment on: Spray pump only: + Cooling tower fan on:

7.00 7.00 7.00

o

The cooling tower fan will be off. The spay pump will be cycling on/off and the supply temperature will vary between 14.2 and 20.0 oC Pretoria - 1369 m above sea level 1.01 kg/cum nominal density

16.6 C CTPACK

Outside Air - 7 cum/s

16.6 C

7 cum/s

20.0 Cdb/14.0 Cwb/43.8 kJ/kg 14.7/12.0

14.2/13.9 43.7 kJ/kg

7 cum/s

38.3 kJ/kg

HPACK COIL

3.0 l/s 13.5 C

Psychrometrics and Heat Balances DB (oC) 3 COIL 7.0 m /s Air in: 20.0 Air out: 14.7 Δenthalpy: Cooling: 7.0 3 HPACK 7.0 m /s Air in: 14.7 Air out: 14.2 Δenthalpy: Heat rejection: 7.0 Water - COIL

COMPACT SYSTEM (Unit 1_partload1)

WB RH (oC) (%) (Cooling Coil) 14.0 53 12.0 75

AH (kg/kg)

DP (oC)

0.0093 0.0093

10.3 10.3

3 m3/s x 1.01 kg/m x (Humidifier Pack) 12.0 75 0.0093 13.9 97 0.0116

m3/s x

3.0 kg/s @

3 1.01 kg/m x

in: out:

Enth (kJ/kg)

Sp.M (kg/m3)

43.8 38.3 5.5 5.5 kJ/kg =

1.01 1.02

10.3 13.7

38.3 43.7 5.5 5.5 kJ/kg =

o 13.5 C o 16.6 C -3.1 degC x

=

38.6 kW 1.02 1.02 38.5 kW

4.19 kJ/kg-degC -38.4 kW

All the cooling collected in the cooling coil is returned in the direct evaporator (humidifier) pack.

Page 16 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

10 m3/s unit running at 80% with the ouside conditions at 30.0 Cdb/18.5 Cwb

Next Example:

m3/s

Psychrometrics:

DB ( C)

WB (oC)

RH (%)

AH (kg/kg)

DP ( C)

Enth (kJ/kg)

Sp.M (kg/m3)

30.0 18.7 16.9

18.5 18.3 16.5

34 96 96

0.0109 0.0155 0.0137

12.7 18.1 16.3

58.2 58.2 51.8

0.97 1.00 1.01

o

Unit on full outside air No cooling equipment on: Spray pump only: + Cooling tower fan on:

8.00 8.00 8.00

o

The spray pump will remain on continously. The cooling tower will cycle and the temperature will vary between between 16.9 and 18.7 oC.

10 cum/s 21.4 Cwb 70.2 kJ/kg

Pretoria - 1369 m above sea level 1.01 kg/cum nominal density

23.3 C 10 cum/s CTPACK

Outside Air - 18 cum/s

19.2 C

8 cum/s

30.0 Cdb/18.5 Cwb/58.2 kJ/kg 19.1/15.0

16.9/16.4 51.8 kJ/kg

8 cum/s

47.0 kJ/kg

HPACK COIL

3.0 l/s 16.1 C

Psychrometrics and Heat Balances DB (oC) 3 COIL 8.0 m /s Air in: 30.0 Air out: 19.1 Δenthalpy: Cooling: 8.0 3 HPACK 8.0 m /s Air in: 19.1 Air out: 16.9 Δenthalpy: Heat rejection: 8.0

COMPACT SYSTEM (Unit 1_Partload2)

WB RH (oC) (%) (Cooling Coil) 18.5 34 15.0 66

AH (kg/kg)

DP (oC)

0.0109 0.0109

12.7 12.7

3 m3/s x 1.01 kg/m x (Humidifier Pack) 15.0 66 0.0109 16.5 96 0.0137

m3/s x

3 1.01 kg/m x

3 CtPACK 10.0 m /s (Humidifier Pack) Air in: 30.0 18.5 34 0.0109 Air out: 19.7 19.7 100 0.0171 Δenthalpy: 3 3 Heat rejection: 10.0 m /s x 1.01 kg/m x

Page 17 of 49

Enth (kJ/kg)

Sp.M (kg/m3)

58.2 47.0 11.3 11.3 kJ/kg =

0.97 1.01

12.7 16.3

1.01 1.01

47.0 51.8 4.9 4.9 kJ/kg =

12.7 19.7

58.2 63.3 5.1 5.1 kJ/kg =

90.6 kW

39.3 kW

0.97 0.99 51.2 kW

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

Water - COIL

Water - HPACK

Water - CTPACK

3.0 kg/s @

3.0 kg/s @

3.0 kg/s @

in: out:

in: out:

in: out:

o 16.1 C o 23.3 C -7.2 degC x

=

4.19 kJ/kg-degC -90.6 kW

=

4.19 kJ/kg-degC 39.3 kW

=

4.19 kJ/kg-degC 51.3 kW

o 19.2 C o 16.1 C 3.1 degC x

o 23.3 C o 19.2 C 4.1 degC x

SUMMARY If used as part of a conventional air-conditioning system, the 2-stage evaporative cooling unit must be controlled to supply air at a given temperature (and pressure). The switching ON/OFF of pumps and cooling tower fans as described above has been the standard method up til now, and, in our experience, rather successful. There are however some valid objections: 1 There are large supply air temperature swings: Ambient air @ 20.0oCdb/14.0oCwb - Split Unit between 14.6 and 20.0 oC between 14.2 and 20.0 oC Compact unit o o Ambient air @ 30.0 Cdb/18.5 Cwb - Split Unit between 16.4 and 19.7 oC between 16.9 and 18.7 oC Compact unit 2 It takes time for a pack to be saturated with water when the pump is switched on and it takes time for all the water to disappear (evaporate and drain away) when the pump is switched off. This introduces both inertia (good?) and a time delay (overshoot - bad?). 3 The continuous wetting and drying of the pack shortens the life if the media. 4 The continuous starting and stopping of pumps and cooling tower fans shortens the life of the motors. Although we have run many 2-stage evaporative cooling units on a START/STOP basis over many years without too many serious problem (we have lost one cooling tower fan motor), we have investigated if it is possible to achieve smooth, accurate temperature control.

Page 18 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

FACE-& BYPASS DAMPER for TEMPERATURE CONTROL

Starting with the SPLIT-TYPE unit.

COOLING TOWER FACE/BYPASS DAMPER

HPACK

OUTS/A DAMPER

SUPPLY FAN

RET/A DAMPER COIL

RETURN FAN

REL/A DAMPER

Example 1:

10 m3/s unit running at 70% with the ouside conditions at 20.0 Cdb/14 Cwb

With the cooling tower OFF and the spray pump ON:

m3/s

DB ( C) 20.0 14.6 17.5 o

Coil leaving: HPACK leaving: Supply Air (mixing):

54% 46%

3.8 3.2 7.0

Page 19 of 49

WB (oC) 14.0 13.9 14.0

AH (kg/kg) 0.0093 0.0115 0.0103

Enth (kJ/kg) 43.8 (by-pass) 43.8 43.8

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

Example 2:

10 m3/s unit running at 80% with the ouside conditions at 30.0 Cdb/18.5 Cwb

The outside air wet bullb is above 17 oC, therefore 2 stages are required. Spray pump ON; Cooling tower On.

m3/s Coil leaving: HPACK leaving: Supply Air (mixing):

Heat Balances: Cooling (Primary air)

20% 80%

1.6 6.4 8.0

DB (oC) 21.7 16.4 17.5

AH (kg/kg) 0.0109 0.0130 0.0126

Enth (kJ/kg) 49.6 (by-pass) 49.6 49.6

In Out

3 8.0 m /s @ 3 8.0 m /s @

58.2 kJ/kg 49.6 kJ/kg 69.3 kW

In Out

3 10.0 m /s @ 3 10.0 m /s @

58.2 kJ/kg 65.1 kJ/kg -69.4 kW

Cooling Capacity: Cooling Tower

WB (oC) 15.9 15.8 15.8

Heat rejected

The COMPACT Unit The compact unit is again more complicated. Switching the cooling tower fan off does not de-activate the cooling coil: In other words, there is heat exchange with the cooling coil, but all the heat is to be returned in the evaporator pack. As seen above, for this example and without by-pass, the air leaves the cooling coil at a temperature of 14.7 o C and the humidfier pack at 14.2 oC. How do we expect to mix 14.7 and 14.2 and achieve 17.5oC? The fact is that, with a compact unit, everything in interrelated: By bypassing part of the air past the humidifier pack, the amount of water cooling is reduced which reduces the amount of cooling through the coil. This increases the temperature of the air leaving the coil, and of the air leaving the humidifier pack and as a result, the mixing temperature:

Page 20 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

FROM : NO BY_PASS (Example 1) Pretoria - 1369 m above sea level 1.01 kg/cum nominal density

16.6 C CTPACK

Outside Air - 7 cum/s

16.6 C

7 cum/s

20.0 Cdb/14.0 Cwb/43.8 kJ/kg 14.7/12.0

14.2/13.9 43.7 kJ/kg

7 cum/s

38.3 kJ/kg

HPACK COIL

3.0 l/s 13.5 C

COMPACT SYSTEM (Unit 1_partload1)

TO: BY-PASS

m3/s Coil leaving: HPACK leaving: ` Supply Air (mixing):

Heat Balances: Cooling Coil

83% 17%

5.8 1.2 7.0

DB (oC) 17.3 18.5 17.5

AH (kg/kg) 0.0093 0.0152 0.0103

Enth (kJ/kg) 41.0 (by-pass) 57.3 43.7

In Out

3 7.0 m /s @ 3 7.0 m /s @

43.8 kJ/kg 41.0 kJ/kg 19.6 kW

In Out

3 1.2 m /s @ 3 1.2 m /s @

41.0 kJ/kg 57.3 kJ/kg -19.6 kW

Cooling Capacity: Humidifier PACK

WB (oC) 13.0 18.1 13.9

Reheat in PACK:

Page 21 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

Example 2:

10 m3/s unit running at 80% with the ouside conditions at 30.0 Cdb/18.5 Cwb

The outside air wet bullb is above 17 oC, therefore 2 stages are required. Spray pump ON; Cooling tower fan ON.

m3/s Coil leaving: HPACK leaving: ` Supply Air (mixing): Heat Balances: Cooling (Primary air)

17% 83%

1.4 6.6 8.0

DB (oC) 19.2 17.1 17.5

AH (kg/kg) 0.0109 0.0152 0.0134

Enth (kJ/kg) 47.1 (by-pass) 57.3 51.7

In Out

3 8.0 m /s @ 3 8.0 m /s @

58.2 kJ/kg 51.7 kJ/kg 52.2 kW

In Out

3 10.0 m /s @ 3 10.0 m /s @

58.2 kJ/kg 63.4 kJ/kg -52.0 kW

Cooling Capacity: Cooling Tower

WB (oC) 15.0 18.1 16.4

Heat rejected

Page 22 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

SUMMARY - CONTROL for Conventional AC Application OADB < 17.5 oC Cooling tower (fan) and spray pump OFF Modulate Return-, Relief- and Outside air dampers: Mix outside air (<17.5 oC) with return air (>17.5 o) to achieve the design supply air temperature (17,5 oC). OADB > 17.5 oC (and OAWB < 17.0 oC) Dampers on full outside air Spray pump ON. Cooling tower (fan) OFF Face/bypass dampers modulate to achieve 17.5 oC OAWB > 17.0 oC Dampers on full outside air Spray pump ON. Cooling tower (fan) ON Face/bypass dampers modulate to achieve 17.5 oC

Page 23 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

2-TEMPERATURE UNITS

Some interesting characteristics of he compact unit can best be illustrated with the following cooling coil/humidifier pack combination:

kJ/kg

24.6 C

7.0 l/s

24.6 C

83.4

Heat Rejection

79.6 72.7 65.4

Cooling

3 20 m /s

20.2 Cdb/

59.6

15.7 Cwb

32.8 Cdb/ 19.6 Cwb

55.7

49.1 kJ/kg

62.1 kJ/kg

53.1 51.5 50.5 49.9 15.7 C 15.7 C

Top 40% of the pack: The leaving enthalpy is higher than the ambient enthalpy Average leaving enthalpy: 75.3 kJ/kg Ambient enthalpy 62.1 kJ/kg 3 Heat rejection: 40% of 20 m /s x 13.1 kJ/kg = This is the 'Cooling Tower' Bottom 60% of the pack: The leaving enthalpy is lower than the entering enthalpy Ambient enthalpy 62.1 kJ/kg Average leaving enthalpy: 53.4 kJ/kg 3 Cooling 60% of 20 m /s x 8.8 kJ/kg =

Heat rejection

106 kW

Cooling

106 kW

Leaving Drybulb Temperatures

o

Cdb

Heat Rejection 23.0 Cdb Cooling 1 18.1 Cdb Cooling 2 16.5 Cdb

24.6 C

7.0 l/s

24.6 C

24.9 24.0 22.5 20.6

3 20 m /s

20.2 Cdb/

19.1

15.7 Cwb

32.8 Cdb/ 19.6 Cwb

18.0

49.1 kJ/kg

62.1 kJ/kg

17.3 16.8 16.5 16.3 15.7 C 15.7 C

Page 24 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

The average temperature of the air leaving the cooling section is:

Nett Room Sensible Cooling Capacity (NRSCC), 3 Primary Air 12.0 m /s

o 17.3 Cdb

with fan heat pick-up: and room design temp:

1.0 degC o 23.5 C

Unit leaving Room entering Room condition(*) NRSCC

17.3 16.9 96 0.0141 16.7 53.4 1.01 18.3 17.2 90 0.0141 16.7 54.4 1.00 23.5 19.0 66 0.0144 17.0 60.4 0.99 3 3 12.0 m /s x 1.01 kg/m x (1.012 + 1.89*0.0141) x (23.5-18.3) degC = 64.7 kW 3 NRSCC/Primary Air: 5.4 kW per m /s (*) With a Room Sensible Heat Gain Factor of 0.9

But the cooling section can be split (say 50/50); the higher temperature could go to the interior, the lower temperature to the perimeter: Cooling 1 - NRSCC Primary Air

3 6.0 m /s

with fan heat pick-up: and room design temp:

1.0 degC o 23.5 C

Unit leaving Room entering Room condition(*) NRSCC

18.1 17.7 96 0.0149 17.5 56.1 1.00 19.1 18.0 90 0.0149 17.5 57.2 1.00 23.5 19.4 69 0.0151 17.7 62.2 0.98 6.0 m3/s x 1.01 kg/m3 x (1.012 + 1.89*0.0149) x (23.5-19.1) degC = 27.4 kW 3 NRSCC/Primary Air: 4.6 kW per m /s (*) With a Room Sensible Heat Gain Factor of 0.9

Cooling 2 - NRSCC Primary Air

3

6.0 m /s

with fan heat pick-up: and room design temp:

Unit leaving Room entering Room condition(*) NRSCC

1.0 degC o 23.5 C

16.5 16.1 96 0.0134 15.9 50.6 1.01 17.5 16.4 90 0.0134 15.9 51.7 1.01 23.5 18.5 63 0.0137 16.2 58.5 0.99 6.0 m3/s x 1.01 kg/m3 x (1.012 + 1.89*0.0134) x (23.5-17.5) degC = 37.4 kW 3 NRSCC/Primary Air: 6.2 kW per m /s (*) With a Room Sensible Heat Gain Factor of 0.9

Page 25 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

Such a unit could then look as follows:

The control of the unit requires further investigation.

Page 26 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

EFFECT OF THE COOLING COIL With Cooling Coil o

Cdb

kJ/kg

24.9

83.4

24.0

79.6

22.5

72.7

20.6

65.4

20.2 Cdb/

19.1

59.6

15.7 Cwb

32.8 Cdb/ 19.6 Cwb

18.0

55.7

49.1 kJ/kg

62.1 kJ/kg

17.3

53.1

16.8

51.5

16.5

50.5

16.3

49.9

24.6 C

7.0 l/s

24.6 C

Averages 19.6

15.7 C 62.1

15.7 C

Without Cooling Coil

o

3 20 m /s

(Straight 1-stage evaporative cooling)

Cdb

kJ/kg

19.9

62.1

19.9

62.1

19.9

62.1

19.9

62.1

19.9

62.1

32.8 Cdb/ 19.6 Cwb

19.9

62.1

62.1 kJ/kg

19.9

62.1

19.9

62.1

19.9

62.1

19.9

62.1

19.4 C

Averages 19.9

62.1

19.4 C

There is an analogy with optics here: In optics, a prism ( or a diffraction grating) can be used to break light up in its constituent spectral colours. Something similar happens with the coil/pack combination: It breaks up the air in its constituent enthalpy levels. This allows us to select what we can use and reject the rest.

Page 27 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

DEWPOINT COOLERS A new trend in evaporative cooling is 'dewpoint cooling': Indirect evaporative cooling, achieving temperatures below the ambient wetbulb without adding any humidity to the air. Examples: The 'Coolerado' (2 oo's!) from Idalex - Colerado USA The OASys (Munters) Oxycell (from Oxycom - Netherlands) All are heavily protected by patents and extremely expensive - They cost more than conventional airconditioning. All seem to be based on plate heat exchangers (technical details are scarce). The 'Coolerado' was named by Maisotsenko of the famous Maisotsenko Cycle. Treated air is progessively diverted to the cooling tower side (the process side) where it absorbs heat and is the disgarded. This is a multistage process and what is left at the end (about 50%) is called the 'Product' air. The principle (and only the principle) can the be reconstructed as follows:

Pretoria - 1369 m above sea level 3 1.01 kg/m nominal density 17.8 Cwb 56.6 kJ/kg

19.3 Cwb 61.8 kJ/kg

20.9 Cwb 68.3 kJ/kg

22.8 Cwb 76.3 kJ/kg

23.0 Cwb 86.3 kJ/kg

27.5 Cwb 99.1 kJ/kg 3

6

4

3

1.5 l/s

29.2/18.5

2

28.6 C

25.8 C

23.4 C

Outside Air - 10 m3/s

9 m /s

26.2/17.6

3

22.7 C

3

8 m /s

23.7/16.8

1 m /s

1.5 l/s

3

21.3 C

1.5 l/s

3

1 m /s

20.0 C

3

7 m /s

21.5/16.1

5

1 m /s

1.5 l/s

3

3

6 m /s

19.7/15.5

21.3 C

18.0 C

18.8 C

1.5 l/s

3

3

5 m /s

Each cooling coil: Each cooling tower:

1 m /s

17.7 C

3

3

5 m /s 18.1/15.0

19.3 C

Etc.

1 m /s

16.7 C

1.5 l/s

Outside Air - 1 m /s 32.8/19.6/62.1

32.8/19.6/62.1/.0113

1

CASCADE UNIT

2 m2 face area, 2 rows, 2.5 mm fin spacing Heat exchange capacity ('AxU'): 4.6 (kW per KJ/kg DeltaEnth)

After Stage 6: Cooling - NRSCC 3 Primary Air 5.0 m /s

with fan heat pick-up: and room design temp:

1.0 degC o 23.5 C

DB WB RH AH DP Enth Sp.M o o ( C) ( C) ( C) (kJ/kg) (kg/m3) (%) (kg/kg) Unit leaving 18.1 15.0 73 0.0113 13.2 46.8 1.01 Room entering 19.1 15.3 68 0.0113 13.2 47.9 1.01 Room condition(*) 23.5 16.9 53 0.0115 13.5 52.9 0.99 3 3 NRSCC 5.0 m /s x 1.01 kg/m x (1.012 + 1.89*0.0113) x (23.5-19.1) degC = 22.9 kW 3 NRSCC/Primary Air: 4.6 kW per m /s o

(*) With a Room Sensible Heat Gain Factor of 0.9

Product air = Primary air: Process air =

3 5.0 m /s from 3 6.0 m /s from

62.1 kJ/kg to 62.1 kJ/kg to

Page 28 of 49

46.8 kJ/kg = 74.7 kJ/kg =

77 kW 76 kW

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

Add more stages … Stage

Prim.Air (m3/s)

DB ( C)

WB (oC)

RH (%)

AH (kg/kg)

DP ( C)

Enth (kJ/kg)

NRSCC(1)

5.00 4.00 3.00 2.00 1.00

18.1 16.8 15.6 14.7 14.0

15.0 14.5 14.1 13.8 13.5

73 79 85 91 95

0.0113 0.0113 0.0113 0.0113 0.0113

13.2 13.2 13.2 13.2 13.2

46.8 45.5 44.3 43.3 42.6

23 kW 24 kW 21 kW 16 kW 9 kW

----6 7 8 9 10 (1)

o

o

(From unit leaving + 1 degC to 23.5 oC)

NRSCC = Nett Room Sensible Cooling Capacity

Add a direct evaporative cooling stage

Pretoria - 1369 m above sea level 3 1.01 kg/m nominal density 17.8 Cwb 56.6 kJ/kg

19.3 Cwb 61.8 kJ/kg

20.9 Cwb 68.3 kJ/kg

22.8 Cwb 76.3 kJ/kg

23.0 Cwb 86.3 kJ/kg

27.5 Cwb 99.1 kJ/kg 3

1.5 l/s

22.7 C

1.5 l/s

3

29.2/18.5

2

28.6 C

25.8 C

23.4 C

Outside Air - 10 m3/s

9 m /s

26.2/17.6

3

1 m /s

1.5 l/s

3

21.3 C

3

8 m /s

23.7/16.8

4

1 m /s

20.0 C

3

7 m /s

21.5/16.1

5

1 m /s

1.5 l/s

3 3

6 m /s

19.7/15.5

19.3 C

18.0 C

18.8 C

3

5 m /s

6

1 m /s

1.5 l/s

3

3

5 m /s 18.1/15.0

21.3 C

3

17.7 C

3

1 m /s

16.7 C

1.5 l/s

Outside Air - 1 m /s 32.8/19.6/62.1

1

32.8/19.6/62.1/.0113

CASCADE UNIT

HPACK

15.3/14.9 46.9 kJ/kg

After Stage 6 + HPAC: 3 Primary Air 5.0 m /s

3

5 m /s

with fan heat pick-up: and room design temp:

1.0 degC o 23.5 C

DB WB RH AH DP Enth Sp.M (oC) (oC) (oC) (kJ/kg) (kg/m3) (%) (kg/kg) Unit leaving 15.3 14.9 96 0.0124 14.7 46.8 1.02 Room entering 16.3 15.2 90 0.0124 14.7 47.9 1.01 (*) Room condition 23.5 17.8 58 0.0127 15.1 56.2 0.99 NRSCC 5.0 m3/s x 1.01 kg/m3 x (1.012 + 1.89*0.0124) x (23.5-16.3) degC = 37.7 kW 3 NRSCC/Primary Air: 7.5 kW per m /s (*) With a Room Sensible Heat Gain Factor of 0.9

Page 29 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

However … The above model of the Maisotsenko Cycle is not very practical: A 6-stage unit consists basically of 6 splt-type evaporative cooling units where the output of one stage is used as the input for the next stage. It would be interesting to see what would happen if we combined the heat exchanging capacities of the 6 stages into one single stage:

Pretoria - 1369 m above sea level 3 1.01 kg/m nominal density

3

5 m /s 23.2 Cwb 78.0 kJ/kg

15.0/14.7

3.5 l/s

15.5 C

3

Outside Air - 10 m3/s

5 m /s

46.3 kJ/kg

17.5/14.8/46.3

26.4 C

3

5 m /s

3

HPACK

5 m /s

26.4 C

32.8/19.6/62.1/.0113

HIGH-CAPACITY UNIT

Cooling Coil: Cooling Tower:

6 x 2 rows = 12 rows 6 x 4.6 kW per kJ/kg DeltaEnth = 28 kW per kJ/kg DeltaEnth

The results are even better than for the Maisotsenko Cycle! (Conclusion?) Without HPAC (Indirect Evaporative Cooling)

Primary Air

with fan heat pick-up: and room design temp:

3 5.0 m /s

1.0 degC o 23.5 C

DB WB RH AH DP Enth Sp.M o o ( C) ( C) ( C) (kJ/kg) (kg/m3) (%) (kg/kg) Unit leaving 17.5 14.8 76 0.0113 13.3 46.3 1.01 Room entering 18.5 15.1 71 0.0113 13.3 47.3 1.01 Room condition(*) 23.5 16.9 53 0.0115 13.6 53.1 0.99 3 3 NRSCC 5.0 m /s x 1.01 kg/m x (1.012 + 1.89*0.0113) x (23.5-18.5) degC = 26.2 kW 3 NRSCC/Primary Air: 5.2 kW per m /s o

(*) With a Room Sensible Heat Gain Factor of 0.9

Page 30 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

With HPAC (Indirect/Direct Evaporative Cooling)

Primary Air

with fan heat pick-up: and room design temp:

3 5.0 m /s

1.0 degC o 23.5 C

DB WB RH AH DP Enth Sp.M o o ( C) ( C) ( C) (kJ/kg) (kg/m3) (%) (kg/kg) Unit leaving 15.0 14.7 97 0.0123 14.6 46.3 1.02 Room entering 16.0 15.1 91 0.0123 14.6 47.3 1.01 Room condition(*) 23.5 17.7 58 0.0126 15.0 55.9 0.99 3 3 NRSCC 5.0 m /s x 1.01 kg/m x (1.012 + 1.89*0.0123) x (23.5-16.0) degC = 38.9 kW 3 NRSCC/Primary Air: 7.8 kW per m /s o

(*) With a Room Sensible Heat Gain Factor of 0.9

Heat Balances: Primary air

In Out

3 5.0 m /s @ 3 5.0 m /s @

62.1 kJ/kg 46.3 kJ/kg 79.8 kW

In Out

3 5.0 m /s @ 3 5.0 m /s @

62.1 kJ/kg 78.0 kJ/kg 79.8 kW

In Out

3.5 l/s @ 3.5 l/s @

In Out

3 5.0 m /s @ 3 5.0 m /s @

In Out

3 10.0 m /s @ 3 10.0 m /s @

Cooling Capacity: Heat rejected

Cooling tower

Water

Heat rejected Air Heat rejected Cooling coil Cooling Capacity:

Page 31 of 49

26.4 15.5 159.6 46.3 78.0 159.6

o

C C kW kJ/kg kJ/kg kW

o

62.1 kJ/kg 46.3 kJ/kg 159.6 kW

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

COMPARISON 3 UNIT TYPES Ambient o

Design conditions

Cdb

Air Temperature - Leaving Unit 2-stage (Unit 1) 2½-stage (Unit 3a) 3-stage (Unit 7) o o o o Cwb Cdb Cdb Cdb Eff Eff Eff

32.8

19.6

17.9

113%

16.9

121%

15.7

130%

19.8 20.7 22.8 24.6 25.8 26.6 27.4 28.7 29.2 29.0 23.8 23.0 22.1

17.3 18.6 19.3 19.9 20.0 19.6 19.2 20.0 20.0 19.7 17.0 16.0 16.9

17.0 18.3 18.9 19.3 19.3 18.7 18.2 18.9 18.8 18.5 16.1 15.1 16.2

113% 113% 112% 112% 112% 113% 113% 113% 113% 113% 113% 113% 113%

16.8 18.2 18.6 19.0 18.9 18.2 17.5 18.3 18.2 17.8 15.5 14.4 15.8

121% 120% 120% 120% 119% 120% 120% 120% 120% 120% 122% 123% 122%

16.5 18.0 18.3 18.6 18.4 17.6 16.8 17.5 17.4 17.0 14.9 13.9 15.3

130% 129% 128% 128% 128% 128% 129% 128% 128% 129% 131% 130% 131%

"Bad" (Muggy) day 7 8 9 10 11 12 13 14 15 16 17 18 19

113%

121%

129%

Typical hot day 7 8 9 10 11 12 13 14 15 16 17 18

20.0 23.7 25.7 27.6 28.6 30.1 30.6 31.6 32.4 33.3 31.6 31.5

17.3 19.0 19.2 19.7 20.2 20.0 19.7 20.1 20.2 19.3 17.6 17.8

16.9 18.4 18.3 18.7 19.1 18.7 18.4 18.6 18.6 17.6 15.8 16.0

16.7 18.0 17.8 18.1 18.5 17.9 17.5 17.7 17.7 16.5 14.7 15.0

16.5 17.6 17.3 17.4 17.7 17.1 16.6 16.7 16.6 15.3 13.5 13.8

7 8 9 10 11 12 13 14 15 16 17 18

22.2 25.2 27.9 29.2 30.9 32.3 33.3 34.2 34.5 34.2 33.3 30.9

17.5 18.7 20.2 20.5 20.9 19.9 19.9 20.3 20.2 19.7 19.3 18.1

16.9 17.9 19.2 19.4 19.6 18.3 18.2 18.5 18.4 17.9 17.6 16.5

16.5 17.4 18.6 18.7 18.8 17.3 17.1 17.4 17.3 16.7 16.5 15.5

16.1 16.9 17.9 18.0 17.9 16.3 16.0 16.2 16.0 15.5 15.3 14.4

Very hot day

Page 32 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

May 2010: The 2-stage evap cooling unit as a source of cold water: The conventional unit (Unit 1):

10 cum/s 21.2 Cwb 69.4 kJ/kg

Pretoria - 1369 m above sea level 1.01 kg/cum nominal density

26.3 73.1 kW

10 cum/s

Outside Air - 20 cum/s

20.5 C

32.8 Cdb/19.6 Cwb/62.1 kJ/kg

116.6 kW 43.5 kW

10 cum/s

21.6/16.2

17.9/17.3 54.9 kJ/kg

10 cum/s

50.6 kJ/kg

3.0 l/s 17.0

STANDARD 2-ST EVAP COOLING UNIT

Nett Room Sensible Cooling Capacity (NRSCC),

Unit leaving Room entering Room condition(*) NRSCC (*)

with fan heat pick-up: and room design temp:

1.0 degC o 23.5 C

17.9 17.3 95 0.0145 17.1 54.8 1.00 18.9 17.6 89 0.0145 17.1 55.9 1.00 23.5 19.0 67 0.0145 17.1 60.7 0.99 10.0 m3/s x 1.01 kg/m3 x (1.012 + 1.89*0.0145) x (23.5-18.9) degC = 48.3 kW

Room latent heat gain ignored.

This is the nett useable cooling capacity produced by this unit. Heat Balances; DB ( C) o

WB (oC)

3 Cooling coil 10.0 m /s Air in: 32.8 19.6 Air out: 21.6 16.2 Δenthalpy: 3 Cooling: 10.0 m /s x 3 Humidifier pack 10.0 m /s Air in: 21.6 16.2 Air out: 17.9 17.3 Δenthalpy: 3 Reheat in Pack: 10.0 m /s x

RH (%)

AH (kg/kg)

DP ( C)

29 58

0.0113 0.0113

13.3 13.3

3 1.01 kg/m x

58 95

0.0113 0.0145

3 1.01 kg/m x

Enth (kJ/kg)

Sp.M (kg/m3)

62.1 50.5 11.6 11.6 kJ/kg =

0.96 1.00

13.3 17.1

1.00 1.00

o

50.5 54.8 -4.3 -4.3 kJ/kg =

Nett cooling:

3 Cooling tower 10.0 m /s Air in: 32.8 19.6 Air out: 21.5 21.2 Δenthalpy: 3 Heat rejection: 10.0 m /s x

116.7 kW

-43.3 kW 73.4 kW

29 98

0.0113 0.0188

3 1.01 kg/m x

Page 33 of 49

13.3 21.1

62.1 69.4 -7.3 -7.3 kJ/kg =

0.96 0.99 -73.0 kW

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

o Now: Notice the low temperature of the water leaving the humidifier pack: 17.0 C.

Investigate if it is feasible to make use of this cooling source:

10 cum/s 21.6 Cwb 71.0 kJ/kg

Pretoria - 1369 m above sea level 1.01 kg/cum nominal density

27.8 89.5 kW

10 cum/s

Outside Air - 20 cum/s

20.7 C

32.8 Cdb/19.6 Cwb/62.1 kJ/kg

89.9 kW 38.1 kW

10 cum/s

24.2/17.0

18.7/18.0 57.0 kJ/kg

10 cum/s

53.2 kJ/kg

3.0 l/s 17.6

3.0 l/s 20.6

2-ST EVAP COOLING UNIT/COOLING TOWER

LOAD 17.6

20.6 37.7 kW

The water is diverted to a secondary cooling load, such a a cooled slab Nett Room Sensible Cooling Capacity (NRSCC),

Unit leaving Room entering Room condition(*) AIR: WATER:

with fan heat pick-up: and room design temp:

1.0 degC o 23.5 C

18.7 18.0 94 0.0150 17.6 57.0 1.00 19.7 18.2 88 0.0150 17.6 58.0 1.00 23.5 19.4 69 0.0150 17.6 62.0 0.98 10.0 m3/s x 1.01 kg/m3 x (1.012 + 1.89*0.0150) x (23.5-19.7) degC = 40.1 kW 3.0 l/s x 4.19 kJ/kg-degC x (20.6-17.6) degC = 37.7 kW

TOTAL Nett Room Sensible Cooling Capacity: (*)

77.8 kW 161% of standard

Room latent heat gain ignored.

This is the nett useable cooling capacity produced by this unit.

Heat Balances;

Cooling coil Air in: Air out: Δenthalpy:

DB (oC)

WB (oC)

RH (%)

AH (kg/kg)

DP (oC)

Enth (kJ/kg)

Sp.M (kg/m3)

3 10.0 m /s 32.8 24.2

19.6 17.0

29 50

0.0113 0.0113

13.3 13.3

62.1 53.2 9.0

0.96 0.99

Page 34 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

3 Cooling: 10.0 m /s x 3 Humidifier pack 10.0 m /s Air in: 24.2 17.0 Air out: 18.7 18.0 Δenthalpy: 3 Reheat in Pack: 10.0 m /s x

Water loop

3 1.01 kg/m x

50 94

0.0113 0.0150

3 1.01 kg/m x

9.0 kJ/kg = 13.3 17.6

53.2 57.0 -3.8 -3.8 kJ/kg =

90.1 kW 0.99 1.00 -38.1 kW

3.0 l/s

37.7 kW

Nett cooling:

89.8 kW

3 Cooling tower 10.0 m /s Air in: 32.8 19.6 Air out: 21.6 21.6 Δenthalpy: 3 Heat rejection: 10.0 m /s x

29 100

0.0113 0.0194

3 1.01 kg/m x

Page 35 of 49

13.3 21.6

62.1 71.1 -8.9 -8.9 kJ/kg =

0.96 0.98 -89.8 kW

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

March 2011: Convert to indirect evaporative cooling Pretoria - 1369 m above sea level 1.01 kg/cum nominal density 8.0 l/s

25.3 C

25.3 C 10 cum/s 23.2 Cwb 77.7 kJ/kg

PACK 1

10 cum/s 19.3 Cwb 61.9 kJ/kg

20.5 C

COIL 1

10 cum/s

8.0 l/s 17.3 C

Outside Air - 20 cum/s

22.1 / 16.3 51.1 kJ/kg

10 cum/s

PACK 2

32.8 Cdb / 19.6 Cwb / 62.1 kJ/kg

18.7 C

MULTI-STAGE INDIRECT EVAP COOLING 10 cum/s

17.7 / 14.8 46.5 kJ/kg

8.0 l/s COIL 2

18.7 C

Nett Room Sensible Cooling Capacity (NRSCC),

with fan heat pick-up: and room design temp:

1.0 degC o 23.5 C

3

10 m /s Unit leaving Room entering Room condition(*) NRSCC:

18.1 15.0 73 0.0113 13.3 46.9 1.01 19.1 15.3 68 0.0113 13.3 47.9 1.01 23.5 16.8 52 0.0113 13.3 52.5 0.99 3 3 10.0 m /s x 1.01 kg/m x (1.012 + 1.89*0.0113) x (23.5-18.7) degC = 45.8 kW

OVERALL Heat Balance DB ( C)

WB (oC)

RH (%)

AH (kg/kg)

DP ( C)

COOLING Air in: Air out: Δenthalpy: Cooling:

3 10.0 m /s 32.8 18.1

19.6 15.0

29 73

0.0113 0.0113

13.3 13.3

HEAT Rejection Air in: Air out: Δenthalpy: Cooling:

3 10.0 m /s 32.8 24.5

o

3 10.0 m /s x

19.6 23.2

3 10.0 m /s x

3 1.01 kg/m x

29 90

0.0113 0.0208

3 1.01 kg/m x

Page 36 of 49

Enth (kJ/kg)

Sp.M (kg/m3)

62.1 46.9 15.2 15.2 kJ/kg =

0.96 1.01

13.3 22.8

0.96 0.97

o

62.1 77.8 -15.7 -15.7 kJ/kg =

153.3 kW

-157.6 kW

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

Compare room condition with conventional 2-stage evaporative cooling: DB (oC) Conventional NRSCC Room condition(*) Indirect NRSCC Room condition(*) (*)

WB (oC)

RH (%)

AH (kg/kg)

DP (oC)

Enth (kJ/kg)

Sp.M (kg/m3)

3 10.0 m /s 23.5 19.0

67

0.0145

17.1

48.3 kW 60.7 0.99

3 10.0 m /s 23.5 16.8

52

0.0113

13.3

45.8 kW 52.5 0.99

Room latent heat gain ignored.

PLANT LAYOUT

REL/A DAMPER

COOLING TOWER FAN

RETURN FAN

RET/A DAMPER

FILTER BANK

OUTS/A DAMPER

PACK 1

PACK 2

COIL 1

COIL 2

PUMP

SUPPLY AIR FAN

Page 37 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

OPERATION

To control the supply air temperature at 17 oC

OADB > 17 oC :

DAMPERS on FULL OUTSIDE AIR PUMP RUNS COOLING TOWER FAN CYCLES to maintain 17 oC

OADB <= 17:

PUMP OFF COOLING TOWER FAN OFF DAMPERS MIX to maintain 17 oC

SUPPLY AIR FAN:

VARIABLE SPEED to maintain CONSTANT RISER PRESSURE (Interference from cycling cooling tower fan?)

Page 38 of 49

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

ENERGY AND WATER CONSUMPTION - COMPARISON WITH CONVENTIONAL AC Based on DEAT2

Variable Volume & VRF Heat recovery 7 Days/week Climate Data - Pretoria 12 hr Operation Max Drawn Unit cons VA/m2 kWh/m2 p.a.

BETTER ENERGY DATA AVAILABLE WATER DATA INCORRECT 17 29.7

VRF Heat Recovery 2-Stage Evap Cooling Water cooled CHW - Spray Econ Water cooled CHW - Dry Econ Water cooled CHW - No Econ Air cooled CHW - Spray Econ Indirect 2-Stage Evap Cooling Air cooled CHW - Dry Econ Air cooled CHW - No Econ

32 33 32 32 33 33 32 32

CONDITIONS ACHIEVED

Max demand kVA/m2 p.a.

Water l/s-m2 p.a.

0.15 0.22 0.25 0.24 0.26 0.28 0.25 0.28 0.30

0 259 277 261 295 88 259 0 0

30.3 35.7 39.3 42.5 42.5 43.9 48.8 53.1

DEAT 2 Rooms 7 Days/week Climate Data - Pretoria

Supply air temperature achieved 2-stage evap 3432 344 10.0% 238 6.9% 136 4.0% 82 2.4% 24 0.7%

Number of operating hours per year No. of hours TAS >17.0 No. of hours TAS >17.5 No. of hours TAS >18.0 No. of hours TAS >18.5 No. of hours TAS >19.0

Ind 2-stage evap 3432 490 hrs 14.3% 392 hrs 11.4% 255 hrs 7.4% 133 hrs 3.9% 65 hrs 1.9%

Room temperatures achieved ROOM

2-stage evap

Ind 2-stage evap

3432 operating hrs per year

3432 operating hrs per year

>23.0

>23.5

>24.0

>23.0

>23.5

>24.0

CB_NO

0 hrs

0 hrs

0 hrs

0 hrs

0 hrs

0 hrs

CB_EA

7 hrs

3 hrs

0 hrs

24 hrs

7 hrs

3 hrs

CB_SO

20 hrs

0 hrs

0 hrs

37 hrs

7 hrs

0 hrs

CB_WE

7 hrs

0 hrs

0 hrs

24 hrs

14 hrs

7 hrs

CB_INT

7 hrs

0 hrs

0 hrs

14 hrs

0 hrs

0 hrs

SB_NO

7 hrs

0 hrs

0 hrs

0 hrs

0 hrs

0 hrs

SB_EA

0 hrs

0 hrs

0 hrs

3 hrs

0 hrs

0 hrs

SB_SO

7 hrs

0 hrs

0 hrs

34 hrs

7 hrs

3 hrs

SB_WE

0 hrs

0 hrs

0 hrs

3 hrs

3 hrs

0 hrs

SB_INT

10 hrs

0 hrs

0 hrs

24 hrs

0 hrs

0 hrs

NB_NO

0 hrs

0 hrs

0 hrs

10 hrs

0 hrs

0 hrs

NB_EA

0 hrs

0 hrs

0 hrs

0 hrs

0 hrs

0 hrs

NB_SO

10 hrs

7 hrs

0 hrs

27 hrs

7 hrs

0 hrs

NB_WE

0 hrs

0 hrs

0 hrs

7 hrs

0 hrs

0 hrs

NB_INT

0 hrs

0 hrs

0 hrs

31 hrs

3 hrs

0 hrs

Average R/A relative humidity 2-stage evap

Ind 2-stage evap

3432 operating hrs per year Hours %

3432 operating hrs per year

>60%

>65%

>70%

>75%

>80%

>60%

>65%

>70%

>75%

1 529

1 202

558

126

7

814

371

109

20

3

44.5%

35.0%

16.3%

3.7%

0.2%

23.7%

10.8%

3.2%

0.6%

0.1%

Page 39 of 49

>80%

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

MOST EXTREME WEEK: 4 to 10 February 2007

oadb

oawb

abshum

max

33.1

22.2

0.0183

min

17.5

15.1

0.0092

average

24.4

18.7

0.0136

(PS Those conditions are not concurrent)

Supply air temperature achieved 2-stage evap 84 45 53.6% 41 48.8% 25 29.8% 17 20.2% 13 15.5% 10 11.9%

Number of operating hours No. of hours TAS >17.0 No. of hours TAS >17.5 No. of hours TAS >18.0 No. of hours TAS >18.5 No. of hours TAS >19.0 No. of hours TAS >19.5 Room temperatures achieved

2-stage evap ROOM

84 operating hrs >23.0

>23.5

>24.0

>24.5

>25.0

CB_NO

2 hrs

1 hrs

1 hrs

1 hrs

0 hrs

CB_EA

14 hrs

8 hrs

4 hrs

2 hrs

0 hrs

CB_SO

11 hrs

10 hrs

3 hrs

2 hrs

0 hrs

CB_WE

8 hrs

7 hrs

6 hrs

3 hrs

0 hrs

CB_INT

13 hrs

10 hrs

5 hrs

1 hrs

0 hrs

SB_NO

6 hrs

4 hrs

1 hrs

1 hrs

0 hrs

SB_EA

14 hrs

9 hrs

6 hrs

1 hrs

0 hrs

SB_SO

13 hrs

8 hrs

6 hrs

2 hrs

0 hrs

SB_WE

10 hrs

6 hrs

3 hrs

3 hrs

1 hrs

SB_INT

16 hrs

10 hrs

6 hrs

0 hrs

0 hrs

NB_NO

7 hrs

5 hrs

2 hrs

1 hrs

0 hrs

NB_EA

13 hrs

10 hrs

5 hrs

0 hrs

0 hrs

NB_SO

12 hrs

6 hrs

5 hrs

1 hrs

0 hrs

NB_WE

8 hrs

5 hrs

3 hrs

2 hrs

0 hrs

NB_INT

16 hrs

10 hrs

6 hrs

1 hrs

0 hrs

AVERAGE (RETURN AIR)

21 hrs

7 hrs

2 hrs

0 hrs

0 hrs

Average R/A relative humidity 2-stage evap 3432 operating hrs per year >60%

>65%

>70%

>75%

>80%

Hours

84 hrs

74 hrs

37 hrs

14 hrs

2 hrs

%

100%

88%

44%

17%

2%

ANOTHER BUILDING Based on CELL C

Variable Volume & VRF Heat recovery 7 Days/week Climate Data - Pretoria 12 hr Operation Max Drawn Unit cons 2 VA/m kWh/m2 p.a.

BETTER ENERGY DATA AVAILABLE WATER DATA INCORRECT

VRF Heat Recovery 2-Stage Evap Cooling Air cooled CHW - Spray Econ Air cooled CHW - No Econ

27 45 54 53

44.3 39.5 76.4 90.6

Page 40 of 49

Max demand kVA/m2 p.a.

Water l/s-m2 p.a.

0.24 0.30 0.51 0.53

0 344 105 0

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

ANOTHER LOCATION Based on CELL C

Variable Volume & VRF Heat recovery 7 Days/week Climate Data - Johannesburg 12 hr Operation Max Drawn Unit cons VA/m2 kWh/m2 p.a.

BETTER ENERGY DATA AVAILABLE WATER DATA INCORRECT

VRF Heat Recovery 2-Stage Evap Cooling Air cooled CHW - Spray Econ Air cooled CHW - No Econ

23 55 61 60

Max demand kVA/m2 p.a.

Water l/s-m2 p.a.

0.23 0.36 0.52 0.53

0 219 130 0

39.5 42.2 66.2 82.9

16 Oct 2012 - COMPACT vs SPLIT + WATER CONSUMPTION Split Unit Process air 10.0 m3/s

25.8 C

21.7 Cwb 71.3 kJ/kg 0.0194 kg/kg Outside air 32.8 Cdb

10.0 m3/s 19.6 Cwb 62.1 kJ/kg 0.0113 kg/kg

5.0 l/s 21.4 C 16.8 C 25.8 C

Product air 3 10.0 m /s

3 10.0 m /s

Outside air 17.3 Cdb/

53.0 kJ/kg

23.9 Cdb 16.9 Cwb 53.0 kJ/kg

16.8 Cwb

0.0113

0.0140 kg/kg

32.8 Cdb

kg/kg

10.0 m3/s

19.6 Cwb 62.1 kJ/kg 0.0113 kg/kg

5.0 l/s

16.8 C

21.4 C 3.0 l/s

Coil

Air Quantity

3 10.0 m /s

In

32.8 Cdb

19.6 Cwb

.0113 kg/kg 62.1 kJ/kg

Out

23.9 Cdb

16.9 Cwb

.0113 kg/kg 53.0 kJ/kg

Water Quantity In

5.0 l/s 21.4 C

Out

25.8 C

Heat balances Air or Water

3 10.0 m /s x

Sens specific heat: 3 10.0 m /s x 5.0 l/s x

3 1.01 kg/m x

(62.1 -

1.012 +

53.0) kJ/kg =

92.3 kW

(1.89 x .0113 kg/kg) = 3 1.01 kg/m x 1.033 x (32.8 -

1.033 kJ/kg-degC 23.9) degC =

92.3 kW

4.19 kJ/kg-degC x

21.4) degC =

92.3 kW

Page 41 of 49

(25.8 -

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

Cooling Tower

3 10.0 m /s

Air Quantity In

32.8 Cdb

19.6 Cwb

.0113 kg/kg 62.1 kJ/kg

Out

21.7 Cdb

21.7 Cwb

.0194 kg/kg 71.3 kJ/kg

Water Quantity

5.0 l/s

In

25.8 C 3 10.0 m /s x

Heat rejected

Out

21.4 C

3 1.01 kg/m x

(71.3 -

62.1) kJ/kg =

92.3 kW

Sensible & Latent components of heat rejection: Sensible

21.7 Cdb

Latent

.0194 kg/kg

from from

32.8 Cdb

-113.1 kW

.0113 kg/kg

205.2 kW

Overall heat rejection:

92.0 kW

Water consumption - Evaporation rate It would appear obvious that the COOLING TOWER evaporation rate must be calculated by dividing the heat rejected (kW) by the heat of evaporation of water (2500 (±) kJ/kg). This would give: 92 kW/

2500 kJ/kg x

3600 =

133 l/hr

This is INCORRECT as will be shown below. The correct way of calculating the evaporation rate is from the increase in absolute humidity of the air over the cooling tower: 3 10.0 m /s x

Evap =

3 1.01 kg/m x

(.0194 -

.0113) kJ/kg x

3600 =

295 l/hr

Alternatively, use the latent component of the heat rejection: 205 kW/

2500 kJ/kg x

3600 =

295 l/hr

The reason for this difference is that the sensible heat content of the cooling tower becomes part of the cooling tower load. (This also applies to cooling towers in conventional AC systems. The difference is that, for conventional systems, the temperatures are much higher and the sensible heat of the CT air can normally be ignored. See below.) Direct Stage

3 10.0 m /s

Air Quantity In

23.9 Cdb

16.9 Cwb

.0113 kg/kg 53.0 kJ/kg

Out

17.3 Cdb

16.8 Cwb

.0140 kg/kg 53.0 kJ/kg

Sensible & Latent components: Sensible Latent

17.3 Cdb .0140 kg/kg

from from

23.9 Cdb

-68.1 kW

.0113 kg/kg

68.0 kW

Total

0.0 kW

All sensible heat is converted into latent 3 Evap = 10.0 m /s x

3 1.01 kg/m x

(.0140 -

.0113) kJ/kg x

3600 =

98 l/hr

Alternatively, using the latent component : 68 kW/ Total water consumption:

2500 kJ/kg x

Evaporation:

3600 =

98 l/hr

Cooling tower: Direct evaporation

295 l/hr 98 l/hr 393 l/hr 785 l/hr

Water consumption (x2 for drift and bleedoff)

Cooling Performance DB ( C) 32.8 17.3 18.3 23.5 o

In (outside air) Out (unit leaving) Room Entering Room condition(*) (*)

WB (oC) 19.6 16.8 17.1 18.7

RH (%) 29 96 90 64

AH (kg/kg) 0.0113 0.0140 0.0140 0.0140

DP ( C) 13.3 16.5 16.5 16.6 o

Enth (kJ/kg) 62.1 52.9 54.0 59.4

Sp.M (kg/m3) 0.96 1.01 1.00 0.99

(1) (2) (3) (4)

Room latent heat gain ignored.

Overall Cooling (1→2): NRSCC (sens 3→4):

9.2 kJ/kg 5.2 degC

1.038 Page 42 of 49

92.6 kW 54.8 kW

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

A LOOK AT COOLING TOWERS IN CONVENTIONAL AC SYSTEMS Cooling towers are normally specified by the heat rejection capacity and the ambient air wetbulb temperature Example A typical specification: 10 l/s from 32.0 C to 26.0 C with an ambient wet bulb temperature of 21.0 C The drybulb temperature is not mentioned, the air quantity normally follows from the selection (about 10 m3/s in this case for a cross-flow cooling tower). Altitude above sea level seems to be of no significance. Heat to be rejected @ 4.19 kJ/kg-degC: 251 kW The evaporation rate is conventionall calculated by dividing the heat rejection rate by the latent heat of evaporation of 2500 kJ/kg. (This is approximate and temperature dependent.) Evaporation rate @ 2500 kJ/kg: 362 l/hr Now consider the real evaporation rate at

an ambient drybulb of an air quantity of altitude above sea level WB RH AH DP (oC) (oC) (%) (kg/kg) 21.0 29 0.0125 14.8

DB (oC) Ambient air 35.0 DeltaEnthalpy over cooling Tower Approx. C/T leaving: 26.3 26.3 Increase in air absolute humidity Evaporation rate Compared to conventional calculation: Sensible over C/T: Latent over C/T

From From

100

0.0259 26.3 0.0133 kg/kg

484 l/hr 134% 35.0 Cdb to 0.0125 kg/kg to

26.3 Cdb 0.0259 kg/kg

Conversely, consider the opposite: An ambient drybulb of 21.0 C Ambient air 21.0 21.0 100 0.0186 21.0 DeltaEnthalpy over cooling Tower Approx. C/T leaving: 26.5 26.5 100 0.0262 26.5 Increase in air absolute humidity 0.0075 kg/kg Evaporation rate Compared to conventional calculation: Sensible over C/T: Latent over C/T

From From

Enth (kJ/kg) 67.5 25 92.5

35.0 C 3 10.0 m /s 1369 m Sp.M (kg/m3) 0.95 kJ/kg 0.96

21.0 Cdb to 0.0186 kg/kg to

68.6 0.99 25 kJ/kg 93.5 0.96

26.5 Cdb 0.0262 kg/kg

Page 43 of 49

-87 kW 338 kW 251 kW

274 l/hr 76% 59 kW 193 kW 251 kW

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

Compact Unit

Process air 10.0 cum/s 21.2 Cwb 69.4 kJ/kg .0189 kg/kg

3.0 l/s

26.3 C

3 10.0 m /s

3 10.0 m /s

20.5 C

Outside air

Product air

32.8 Cdb 3 10.0 m /s

17.6 Cdb/

3 10.0 m /s

54.9 kJ/kg

3 10.0 m /s

21.6 Cdb 16.2 Cwb 50.6 kJ/kg

17.3 Cwb

19.6 Cwb 62.1 kJ/kg .0113 kg/kg

.0113 kg/kg

0.0146 kg/kg 17.0 C

17.0 C

3.0 l/s Coil

3 10.0 m /s

Air Quantity In

32.8 Cdb

19.6 Cwb

.0113 kg/kg 62.1 kJ/kg

Out

21.6 Cdb

16.2 Cwb

.0113 kg/kg 50.6 kJ/kg

Water Quantity In

3.0 l/s 17.0 C

Out

26.3 C

Heat balances 3 10.0 m /s x

Air or

Sens specific heat: 3 10.0 m /s x

Water Cooling Tower

3.0 l/s x

3 1.01 kg/m x

(62.1 -

1.012 +

50.6) kJ/kg =

116.6 kW

(1.89 x .0113 kg/kg) = 3 1.01 kg/m x 1.033 x (32.8 -

1.033 kJ/kg-degC 21.6) degC =

116.6 kW

4.19 kJ/kg-degC x

17.0) degC =

116.6 kW

(26.3 -

3 10.0 m /s

Air Quantity In

32.8 Cdb

19.6 Cwb

.0113 kg/kg 62.1 kJ/kg

Out

21.2 Cdb

21.2 Cwb

.0189 kg/kg 69.4 kJ/kg

Water Quantity In

3.0 l/s 26.3 C

Out

20.5 C

Heat rejected 3 10.0 m /s x

3 1.01 kg/m x

(69.4 -

62.1) kJ/kg =

Difference

73.1 kW 43.6 kW

Sensible & Latent components of heat rejection: Sensible Latent

21.2 Cdb .0189 kg/kg

from from

32.8 Cdb

-117.8 kW

.0113 kg/kg

190.8 kW

Overall heat rejection: water

73.1 kW 3.0 l/s x

4.19 kJ/kg-degC x

Page 44 of 49

(26.3 -

20.5) DegC

73.1 kW

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

Water consumption - Evaporation rate 3 10.0 m /s x

Evap =

3 1.01 kg/m x

(.0189 -

.0113) kJ/kg x

3600 =

274 l/hr

Alternatively, use the latent component of the heat rejection: 191 kW/

Direct Stage

2500 kJ/kg x

3600 =

275 l/hr

3 10.0 m /s

Air Quantity In

21.6 Cdb

16.2 Cwb

.0113 kg/kg 50.6 kJ/kg

Out

17.6 Cdb

17.3 Cwb

.0146 kg/kg 54.9 kJ/kg

Sensible & Latent components: Sensible Latent

17.6 Cdb .0146 kg/kg

from from

21.6 Cdb

-40.4 kW

.0113 kg/kg

83.8 kW

Total Evap =

43.4 kW 3 10.0 m /s x

3 1.01 kg/m x

(.0146 -

.0113) kJ/kg x

3600 =

120 l/hr

Alternatively, using the latent component : 84 kW/ Total water consumption:

2500 kJ/kg x

Evaporation:

3600 =

121 l/hr

Cooling tower:

274 l/hr

Direct evaporation

121 l/hr 395 l/hr

Water consumption (x2 for drift and bleedoff)

790 l/hr

Cooling Performance

In (outside air) Out (unit leaving) Room Entering Room condition(*) (*)

DB (oC) 32.8 17.6 18.6 23.5

WB (oC) 19.6 17.3 17.7 19.1

RH (%) 29 97 91 67

AH (kg/kg) 0.0113 0.0146 0.0146 0.0146

DP (oC) 13.3 17.2 17.2 17.2

Enth (kJ/kg) 62.1 54.9 55.9 61.0

Sp.M (kg/m3) 0.96 1.01 1.00 0.99

(1) (2) (3) (4)

Room latent heat gain ignored.

Overall Cooling (1→2): NRSCC (sens 3→4):

7.3 kJ/kg 4.9 degC

1.040

Page 45 of 49

73.3 kW 51.2 kW

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

20 December 2013 - More MULTI-STAGE EVAPORATIVE COOLING UNITS MULTI-STAGE EVAPORATIVE COOLING JOHANNESBURG

Altitude above sea : Atmospheric pressure : Nominal density :

1694 m 826 mbar 3 0.97 kg/m

218 kW

187 kW 8.0 l/s

22.9 C

3 10.0 m /s o

o

21.4 Cwb 72.0 kJ/kg 0.0198 kg/kg

C T P A C K

20.2 Cdb o 15.4 Cwb 49.5 kJ/kg

C T

0.0115 kg/kg 3 10.0 m /s

C O I L

3 10.0 m /s

8.0 l/s

20.2 15.4 49.5 0.0115

16.4 C 3 10.0 m /s o 14.5 Cdb o 14.3 Cwb 46.2 kJ/kg

0.0125 kg/kg

E V P P C K

o 17.0 Cdb o 14.3 Cwb 46.2 kJ/kg

0.0115 kg/kg 3.0 l/s

o

Cdb Cwb kJ/kg kg/kg

o

A H U C O I L 17.3 C 31 kW

Effectiv. Cooling Heat Rejection

134%

111% 3 10.0 m /s frm 3 10.0 m /s frm

59.1 kJ/kg to 59.1 kJ/kg to

Page 46 of 49

46.2 kJ/kg = 72.0 kJ/kg =

125 kW -124 kW

o 29.5 Cdb o 18.3 Cwb 59.1 kJ/kg .0115 kg/kg 3 20.0 m /s

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

MULTI-STAGE EVAPORATIVE COOLING PRETORIA

1369 m

Altitude above sea : Atmospheric pressure : Nominal density :

859 mbar 3 1.01 kg/m

252 kW

214 kW 8.0 l/s

25.3 C

3 10.0 m /s o

o

23.3 Cwb 78.1 kJ/kg 0.0215 kg/kg

C T P A C K

22.2 Cdb o 16.9 Cwb 53.1 kJ/kg

C T

0.0120 kg/kg 3 10.0 m /s

C O I L

3 10.0 m /s

8.0 l/s

22.2 16.9 53.1 0.0120

17.7 C 3 10.0 m /s o 15.9 Cdb o 15.7 Cwb 49.3 kJ/kg

0.0131 kg/kg

E V P P C K

o 18.6 Cdb o 15.7 Cwb 49.3 kJ/kg

0.0120 kg/kg 3.0 l/s

o

Cdb Cwb kJ/kg kg/kg

o

A H U C O I L 18.9 C 38 kW

Effectiv. Cooling Heat Rejection

133%

111% 3 10.0 m /s frm 3 10.0 m /s frm

63.7 kJ/kg to 63.7 kJ/kg to

Page 47 of 49

49.3 kJ/kg = 78.1 kJ/kg =

145 kW -145 kW

o 32.5 Cdb o 20.0 Cwb 63.7 kJ/kg .0120 kg/kg 3 20.0 m /s

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

MULTI-STAGE EVAPORATIVE COOLING WINDHOEK

1725 m

Altitude above sea : Atmospheric pressure : Nominal density :

823 mbar 3 0.96 kg/m

282 kW

242 kW 8.0 l/s

23.8 C

3 10.0 m /s o

o

21.8 Cwb 74.1 kJ/kg 0.0205 kg/kg

C T P A C K

20.2 Cdb o 13.9 Cwb 44.9 kJ/kg

C T

0.0096 kg/kg 3 10.0 m /s

C O I L

3 10.0 m /s

8.0 l/s

20.2 13.9 44.9 0.0096

15.3 C 3 10.0 m /s o 12.7 Cdb o 12.5 Cwb 40.7 kJ/kg

0.0110 kg/kg

E V P P C K

o 16.2 Cdb o 12.5 Cwb 40.7 kJ/kg

0.0096 kg/kg 3.0 l/s

o

Cdb Cwb kJ/kg kg/kg

o

A H U C O I L 16.5 C 40 kW

Effectiv. Cooling Heat Rejection

136%

111% 3 10.0 m /s frm 3 10.0 m /s frm

57.4 kJ/kg to 57.4 kJ/kg to

Page 48 of 49

40.7 kJ/kg = 74.1 kJ/kg =

161 kW -161 kW

o 32.4 Cdb o 17.8 Cwb 57.4 kJ/kg .0096 kg/kg 3 20.0 m /s

2-STAGE EVAP COOLING UNITS T. Herman 27/10/2009, upd mar 2011, dec 2013

TOON HERMAN ASSOCIATES

A 25 m3/s MULTI-STAGE EVAPORATIVE COOLING UNIT could look as follows:

Primary (Product) air quantity: Cooling Tower (Process) air quantity:

3 25 m /s 3 25 m /s

The unit is very large but can serve a building of 3500 - 5000 m 2. And the design can obviously be improved: Use centrifugal primary fan, reduce process air ratio …

TO BE CONTINUED …

Page 49 of 49