primary cooling coil

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PRIMARY COOLING COILFOR AIR HANDLING UNIT

Author:Engr. K.H., Kong is Mechanic Engineer.(IEM member, No: M21065)Bachelors Degree with Honors with Distinction in Mechanical Engineering.


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Primary Cooling Coil for Air Handling Unit

Copyright 2008 by Kok-Haw KongAll Right Reserved.

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PRIMARY COOLING COIL FOR AIR HANDLING UNIT

ABSTRACT

In this write up, an energy-saving device is introduced into the air-handling

unit (AHU). Due to increases in energy costs, energy saving devices or systems have

become the new preferred trend in tropical countries. A primary cooling coil system

is incorporated into the AHU to recycle the condensate water in order to achieve

energy savings.

The warm outside fresh air is drawn through the primary cooling coil to

recover the latent heat of condensate water. The large temperature gradient between

the warm fresh air and condensate water enlarges the amount of energy recovered.

This system is well applicable to primary AHU which serves primary air to the air-

conditioning system. The cost of installing the device is negligible but in returning

long term saving. Furthermore, the de-cooled condensate water can be drained off

through non-insulated drain pipe.

The idea generation is discussed in the introduction. Chapters 2 and 3 describe

the preliminary calculation of the energy saving percentage. Chapter 4 shows the

detailed theoretical study of the development of the energy saving system. This shows

the viability of the idea. Chapter 5 will recommend the design of the primary cooling

coil configuration to achieve the desired cooling effect and energy saving strategy.

Lastly, the conclusion of the whole study is presented.


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1.0 INTRODUCTION

In tropical countries such as the United Arabia Emirates (UAE) and Malaysia,

the climate is always hot and highly humid throughout the year. Lots of energy is

consumed for Air-Conditioning and Mechanical Ventilation (ACMV) systems.

For buildings such as hotels and hospitals, energy is used to cool down the

fresh air in the primary AHU and pumped into the building to achieve the internal air

quality required. Due to the high humid climate, lots of condensed water is drained

away as a by-product. However, the cooled condensate water actually consumes 2%

of the total cooling load of the system on average. Here, we are studying the recovery

of the condensed water in which the 2% energy can be saved. 2% of energy for a

system running 24 hours a day can make a significant reduction for the operating cost.

The condensed water is first collected by the condensate drain pan below the

main secondary cooling coil. Then, the collected condensed water is pumped into the

primary cooling coil for the de-cooling process.

In order to maximize the energy recovery, a primary cooling coil is

recommended to be installed at the fresh air intake opening of the AHU where the

temperature gradient between the air outside and condensate water is greater, rather

than at the side in which air returns or at the mixing plenum.


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2.0 PRELIMINARY THEORETICAL STUDY

In this study, we assume that the AHU is a chilled-water cooled type with a

modulating control valve. The percentage of energy saved is calculated by comparing

the AHU with and without the primary cooling coil.

Figure 2.1

Figure 2.1 shows a conventional AHU configuration without the primary

cooling coil. From the energy equation and psychometric chart, the cooling load per

unit of dry air is given as

211 H H

m

Q

a

=

(2.1)


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From the mass balance equation, the condensed water per unit of dry air is

( )21. wwm

m

a

w =

(2.2)

For an AHU with a primary cooling coil, the cooling load of the condensate water per

unit of dry air is given as follows:

wwp

a

w

a

T C m

m

m

Q=

,2 on water side, or aap

a

a

a

T C m

m

m

Q=

,2 on air side (2.3)

Rearranging equation (2.3), we simplify it to

( ) ( )oaiaiwowap

wp

a

w T T T T C

C

m

m,,,,

,

, =

(2.4)

For the preliminary study, we first assume owoa T T ,, = . This assumption and

condition depends on the coil length. In fact, iaow T T ,, may be achieved if the coil is

long enough. The temperature difference is thus higher and higher energy saving is

possible.

We also assume 100% sensible heat transfer across the primary cooling coil

for equation (2.4). This is due to low heat transfer and high air velocity. This

assumption will be checked using a psychrometric chart and verified in chapter 3.


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3.0 CASE STUDY

The climate of Malaysia (Tropical Country) is selected for study. Generally,

three cases are studied for comparing the percentage of energy saved. The first two

cases are two extreme conditions while the last case is the normal average condition.

These are:

1) During the Afternoon, the Warmest Period of the Day

2) Under Extreme Conditions, a Hot Heavy Rainy Day, and

3) Under Normal Conditions, Average Weather.

3.1 During Afternoon, the Warmest Period of the Day

During the warmest period, where outside air condition is DB 1/WB 1 =

93.9/78.1 F, and the air off coil condition is DB 2/WB 2 = 55.4/53.6 F


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From the psychometric chart, we obtain H 1 = 41.4 Btu/lbm, w 1 = 0.017 lbv/lba

H2 = 22.3 Btu/lbm, w 2 = 0.008 lbv/lba

The initial calculation is performed to find the overall cooling load of the

secondary cooling coil. From energy equation (2.1), the cooling load per unit of dry

air is

lbmBtuH H m

Q

a

/ 1.193.224.41211 ===

(3.1)

From the mass balance equation (2.2), the condensed water per unit of dry air is

( ) ( ) lbalbvwwm

m

a

w / 009.0008.0017.021. ===

(3.2)

From the energy equation (2.4) across the primary coil, the equilibrium temperature is

calculated as follows:

( ) ( )oaiaiwowap

wp

a

w T T T T C

C

m

m,,,,

,

,=

(3.3)

( ) ( ) owoaoaow T T assumeT T ,,,, ,9.934.5524.01

009.0 ==

0775.29.930375.1 , +=owT

F T owo

5.92, = (3.4)

From energy equation (2.3), the cooling coad of condensate water per unit of dry air is


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( ) lbmBtuT C m

m

m

Qwwp

a

w

a

/ 3339.04.555.921009.0,2 ===

(3.5)

Hence, the actual cooling load required from chilled water is reduced to

lbmBtum

Q

m

Q

aa

/ 77.183339.01.1921 ==

(3.6)

Therefore, the percentage of cooling capacity reduction (the energy saving) is

%75.11.19

3339.0

1

2 ==

QQ (3.7)

3.2 Case Study on the Additional Two Conditions

Similar studies such as those in chapter 3.1 are carried out for two additional

conditions. The computed results are tabulated as follows:

Item Description WarmestPeriod

Hot HeavyRainy Day

AverageWeather

1) Outside air condition, DB 1/WB 1 (F) 93.9/78.1 84.2/81.8 81.5/73.62) Air Off Coil Condition, DB 2/WB 2 (F) 55.4/53.6 55.4/53.6 55.4/53.63) Energy Equation (2.1), (Btu/lbm) 19.1 23.3 14.84) Mass Balance Equation (2.2), (lbv/lba) 0.009 0.015 0.0085) Equilibrium Temperature, T w,o (F) 92.5 82.5 80.76) Energy Equation (2.3), (Btu/lbm) 0.3339 0.4065 0.20247) Actualy Cooling Load, (Btu/lbm) 18.77 22.89 14.60

8) Energy Saving (%) 1.75 1.74 1.37

Referring to the psychometric chart, the cooling effect for the primary cooling

coil is plotted and checked. It is found that the off coil condition of fresh air across the

primary coil is far from the saturation curve. Due to high air flow, 100% of the


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sensible heat transfer is achievable. No condensation occurs at the primary cooling

coil even during rainy days. Therefore, the assumption of energy equation (2.4) is

valid in this case.

In this study, we assume that owoa T T ,, = and the average energy saving of the

primary cooling coil is 1.37%. In the actual case, a higher percentage of energy saved

can be achieved when the coil surface temperature is held constant and maintained at

iaT , . The constant coil surface temperature is due to high air flow across the primary

coil. The condition of iaow T T ,, may be achieved due to the longer coil length.

Higher temperature differences between condened water inlet and the outlet produces

higher energy savings.

4.0 PRIMARY COOLING COIL CONFIGURATION

In this chapter, we study the configuration and the requirements of the primarycooling coil in order to achieve the required cooling condition. We also study the

effect of the static loss in air across the primary cooling coil for fan selection.

From the Reynold 1,3 number, D

mwD

=4

Re (4.1)

where D = diameter of coil,

wm = water mass flow rate, = viscosity of fluid.

For a fully developed turbulent flow in a smooth circular tube, from the

Petukhov, Gnielinski 1,4 correlation, the Nusselt 1,4 number is given as follows


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( )( )( ) ( )

+

+

=

32

32211

1Pr87.121

Pr1000Re8LD

f

f Nu DD (4.2)

where the friction factor given by Petukhov 1,4 ,

( ) 264.1Reln790.0 = Df (4.3)

Pr = Prandtl Number

For cases of constant wall temperature and a fluid with Pr > 0.7, correlation

(4.2) for constant wall heat flux can be used with negligible error. 4

For a preliminary study without knowing the pipe length, we first assume that

the pipe length is much longer than the pipe diameter, L>>D. Therefore, equation (4.2)

reduces to

( )( )( ) ( )1Pr87.121

Pr1000Re83221 +

=

f

f Nu DD (4.4)

The error for the modified equation of (4.4) will be discussed at the end of this

chapter. For the preliminary assumption of the omitted item of equation (4.2), the

ratio of the pipe diameter in millimeters to the pipe length of meters is 1:1000. The

value of the ratio to the power of 2/3 is reduced to 1:100. Hence, the error is within a

1% range of the calculated value.

The Nusselt number also can be expressed as follows for a simplified application:

k hD

Nu D = (4.5)

where h = convection coefficient, D = pipe diameter, k = thermal conductivity of fluid.


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For the heat transfer equation in a circular tube 1, we have

( )lm

iwowwpw

T

T T

DL

C mh

=

,,,

(4.6)

Combining equation (4.5) & (4.6) and rearranging, we obtain

( )lm

iwow

D

wpw

T

T T

kNu

C mL

=

,,,

(4.7)

where

( )iwowiwow

lm

T T

T T T

,,

,,

ln

= (4.8)

iwsiwowsow T T T T T T ,,,, , == (4.9)

From the above equations, our objectives are to generally determine the

following:

1) The required coil configuration to achieve the required cooling effect. The

parameters are coil diameter and length.

2) To check on the static loss in air across the primary cooling coil where higher

static loss in air requires a larger fan and a higher operating cost.

Again, three similar cases are studied to compare the percentage of energy saved. The

3 cases are:

1) During the Afternoon, the Warmest Period of the Day

2) Under Extreme Conditions, a Hot Heavy Rainy Day, and

3) Under Normal Conditions, Average Weather.


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4.1 During Afternoon the Warmest Period,

Taking DB 1/WB 1 = 34.4/25.6 C (93.9 /78.1 F), DB 2/WB 2 = 13/12 C (55.4/53.6 F),

Ts = 34.4 C (93.9 F), T w,o = 33.6 C (92.5 F), and T w,i = 13 C (55.4 F),

kgakgvmm aw 009.0=

(0.009 lbv/lba )

Taking D = 12mm ( 1/ 2), at the mean water temperature = (33.6+13)/2 = 23.3 C

(73.9 F), = 932 x 10 -6 Ns/m 2, Pr = 6.415

For an AHU serving 3000 l/s (6360 cfm) fresh air, the area of fresh air intake

is assumed to be 1300mm(W)x700mm(H). During the warmest period, the calculated

values are,

skgmw / 0314.01614.13009.0 ==

(4.10)

( )( ) 357010932012.00314.044

Re 6 =

==

D

mwD , turbulent flow. (4.11)

The friction factor,

( ) ( )[ ] 043.064.13570ln790.064.1Reln790.0 22 === Df (4.12)

Hence, the Nusselt number,


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( )( )( ) ( )1Pr87.121

Pr1000Re83221 +

=

f

f Nu DD

( )( )( ) ( ) 27283.3

615.88

1415.68043.07.121

415.6100035708043.03221

==+

=DNu (4.13)

From equation (4.9), the temperature difference is

iwsiwowsow T T T T T T ,,,, , ==

C T C T iwowoo

4.21134.34,8.06.334.34 ,, ==== (4.14)

From equation (4.8), the log mean temperature difference is

( ) ( ) C T T T T

T iwow

iwowlm

o

268.64.218.0ln4.218.0

ln ,,

,, =

=

= (4.15)

At a water temperature of 23.3 C, the specific heat of water is C p,w = 4180

J/kgK, the thermal conductivity is k w = 0.6078 W/mK. From equation (4.7), we obtain

the required pipe length,

( ) ( )m

T

T T

kNu

C mL

lm

iwow

D

wpw367.8

268.6276078.0136.3341800314.0,,, =

=

=

(4.16)

For a 12mm ( 1/ 2) diameter primary cooling coil, the maximum coil length

required to achieve a cooling effect of 1.75% of energy is 8.367m (27.45ft). The net

effective primary cooling coil area perpendicular to the air flow direction is

Net Area = pipe length x pipe diameter

= 8.367 x 0.012 = 0.1m 2 (1.08ft 2) (4.17)


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The total opening area for incoming fresh air, A f = 1.3m x 0.7m = 0.91 m2

Hence, the ratio of the effective primary cooling coil area to the fresh air

opening is 0.1/0.91 = 11%, where the pressure drop/static loss across the primary

cooling coil at the fresh air opening is negligible.

4.2 Under Extreme Condition, a Hot Heavy Rainy Day

Similar studies such as those in chapter 4.1 are carried out for a hot heavy

rainy day. The computed results are tabulated as follows:

Item Description WarmestPeriod

Hot HeavyRainy Day

1) Outside air condition, DB 1/WB 1 (C) 34.4/25.6 29/27.72) Air Off Coil Condition, DB 2/WB 2 (C) 13/12 13/123) Coil Surface Temperature, T s (C) 34.4 29.04) Condensate Inlet Temperature, T w,i (C) 13.0 13.05) Equilibrium Temperature, T w,o (C) 33.6 28.066) Mass Balance Equation (2), (kgv/kga) 0.009 0.0157) Cooling Coil Diameter, D (mm) 12 128) Mean Temperature ( C) 23.3 20.59) Viscosity of Fluid, (Ns/m 2) 932 x 10 -6 995 x 10 -6

10) Prandtl Number, Pr 6.415 6.90211) Air Intake Volume Flowrate, m a (l/s) 3000 300012) Condensate Water Flowrate, m w (kg/s) 0.0314 0.052313) Reynold number, Re D 3570 557714) Type of Flow Turbulent Turbulent15) Pipe Friction Factor, f 0.043 0.0373416) Nusselt Number, Nu D 27 4517) Log Mean Temperature Difference, T lm (C) 6.268 5.31318) Specific Heat of Condensate Water, C p,w (J/kg.K) 4180 418219) Thermal Conductivity of Condensate Water, k w

(W/m.K)0.6078 0.6036

20) Length of Cooling Coil, L (m) 8.367 7.26521) Net Effective Obstructed Area, A c (m

2) 0.1 0.0944


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22) Percentage of obstructed area (%) 11 1023) Pressure Drop Across Primary Cooling Coil Negligible Negligible

4.3 Hourly Simulation for Average Weather

An hourly simulation is carried out to study the hourly energy savings

throughout a day. A day in July is selected. The primary cooling coil diameter is taken

to be 8mm ( 3/ 8). For an AHU serving 3000 l/s (6360 cfm) fresh air, the opening area

of fresh air intake is taken to be 1300mm (4.27 ft) (W) x 700mm (2.3 ft) (H), which is

equal to 0.91m 2 (9.8ft 2). Intake air velocity is 3.3m/s (650fpm).

From table 4.1 shown below, for an installed 8mm ( 3/ 8) diameter primary

cooling coil, the maximum coil length required is 6.847 meters (22.466 ft), which able

to achieve 1.42% of energy saved, happened at the hour of 23. The ratio of the

primary cooling coil area to the fresh air opening is 6.77%, where the pressure drop

across primary cooling coil is negligible. The supply fan capacity can remain as is

without additional power to drive the fan.

With the designed length of the primary cooling coil as 8m (26.2 ft), the

primary cooling coil can achieve a higher energy saving level at any time compared to

the calculated value, which is tabulated in table 4.1. The energy saved can be higher

than the average value of 1.51%. Furthermore, , the coil surface temperature is higher


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than the calculated value in actual cases where iaow T T ,, . This is due to the high

convection coefficient of the high velocity of fresh air across the primary cooling coil.

A higher temperature difference between the inlet and outlet for condensed water

contributed to higher energy savings.

Moreover, the energy saved can achieve a higher percentage because more

condensed water is collected on a rainy day.

From equation (4.4), the corrective error is very small. The ratio between a

pipe diameter of 8mm to the pipe length of 8000mm is 1:1000. Hence, the corrective

error is 1% of the calculated value. The energy saved is 1.51% 0.015%.


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Table 4.1An hourly simulation of the average weather to obtain the desired design configuration

Outdoor Original Cooling Primary Cooling Energy Saving Coil Effective Temperature, F Load, Q 1/m a Load, Q 2/m a Percentage Reynold Length Coil Ar

Hour Jul DB Jul WB Btu/lbm Btu/lbm (%) Re (ft) (ft0 80.78 74.48 15.700 0.220 1.40% 4930 18.935 0.55911 79.88 74.30 15.500 0.212 1.37% 4902 19.019 0.56162 79.16 74.12 15.300 0.206 1.35% 4879 19.087 0.56363 78.44 73.94 15.200 0.200 1.31% 4857 19.154 0.56564 77.90 73.76 15.000 0.195 1.30% 4841 19.205 0.56715 77.72 73.76 15.000 0.194 1.29% 4835 19.222 0.56766 78.08 73.76 15.000 0.197 1.31% 4846 19.188 0.56667 78.98 74.12 15.300 0.205 1.34% 4874 19.104 0.56418 80.42 74.48 15.700 0.217 1.38% 4919 18.968 0.56019 82.58 75.02 16.200 0.236 1.46% 4987 18.767 0.5542

10 84.92 75.74 16.800 0.256 1.52% 5064 18.550 0.547811 87.62 76.46 17.500 0.279 1.60% 5156 18.302 0.540412 90.32 77.00 18.000 0.303 1.68% 5244 18.071 0.533613 92.30 77.54 18.500 0.320 1.73% 5305 17.915 0.529014 93.56 77.90 18.900 0.331 1.75% 5345 17.816 0.526115 93.92 78.08 19.100 0.334 1.75% 5357 17.788 0.525316 93.56 77.90 18.900 0.331 1.75% 5345 17.816 0.526117 92.30 77.54 18.500 0.320 1.73% 5305 17.915 0.529018 90.68 77.18 18.200 0.306 1.68% 5255 18.043 0.532819 88.52 76.64 17.700 0.287 1.62% 5187 18.220 0.538020 86.36 76.10 17.200 0.269 1.56% 5112 21.540 0.636121 84.56 75.56 16.700 0.253 1.51% 5052 21.881 0.646122 82.94 75.20 16.300 0.239 1.47% 4999 22.204 0.655723 81.68 74.84 16.000 0.228 1.42% 4958 22.466 0.6634

AVERAGE 16.758 0.256 1.51% MAX 22.466 0.6634


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5.0 THE DESIGN OF PRIMARY COOLING COIL

Figure 5.1 shows the recommended configuration of the primary cooling coil

for the AHU and Figure 5.2 shows the recommended detailed design of the primary

cooling coil.

Figure 5.1


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Figure 5.2

Let us take an example for an AHU serving 3000 l/s (6360 cfm) fresh air

across a 0.91 m 2 (9.8 ft 2) (1.3m x 0.7m) fresh air intake opening with the primary

cooling coil installed as shown in Figures 5.1 and 5.2. Outside air is entering the AHU

at temperature DB/WB = 32.2

C/30

C (90

F/86

F). Air face velocity = 3.3 m/s (650

fpm).

On the water side, condensate water is entering coil at 13

C (55.4

F). The

primary cooling coil diameter = 8mm ( 3/ 8). Condensate water flow rate = 0.0314 l/s

(0.414 igpm), and condensate water velocity = 0.625 m/s (123 fpm).


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5.1 Operating Methodology of Primary Cooling Coil

Referring to Figure 5.2, condensate water is periodically pumped to the

balancing tray. Then, the flow of the condensate water is adjusted by the control valve.

When the balancing tray is filled with water, the fresh condensate water is fed into the

tube by gravity and the de-cooled condensate water will discharge to the nearest floor

trap or scupper drain. When balancing tray is empty, condensate water stays in the

coil until the balancing tray is refilled with fresh condensate water.

When water is flowing through the cooling coil, the heat transfer rate is as per

designed performance. When the balancing tray is empty, the water stays in the

primary cooling coil and maximum heat transfer occurs. The energy saved is higher

than the calculated value.

5.2 Operating Cost of Pump

The condensate pump is used to pump the condensate water from the

condensate drain pan to the balancing tray. For example, when the water level in the

drain pan achieves 3 liters (0.66 igallons), a condensate water pump with a capacity of

1.0 l/s (13.2 igpm) is used to pump away the condensate water within 3 seconds.


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For a 1.0 l/s pump, the power consumption is 120W. The worst case is taken

to calculate the total power consumption of the condensate pump. During extreme

conditions, the total condensate water generated is 222 liters per day.

Pumping time is 222/3 = 74 times per day.

Pumping period is 222 seconds = 0.0617 hrs

Round up to 0.1 hrs. Total electricity consumption required is,

0.12kW x 0.1 hrs = 0.012 kWhr

0.012 kWhr x RM 0.294 (USD 0.077) = RM 0.0035 (USD 0.001) per day

The heat generated from the pump is roughly 42W. By adding the primary

cooling coil to AHU, the de-cooled condensate water can be discharged to the nearest

floor trap or scupper drain without condensation on the drainage pipe.

5.3 Maintenance

The primary cooling coil only requires simple maintenance. A suitable

cleaning chemical is poured into the condensate drain pan. Then, pump it to the

balancing tray, to the primary cooling coil and let it drain out. For the external coil

surface, clean it with a suitable cleaning chemical. The condensate pump may need to

be replaced yearly.


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6.0 CONCLUSION

For a modulating control valve chilled-water cooled type AHU, when the

primary cooling coil is placed at the fresh air side of the AHU, it can achieve

1.5% 0.015% of energy savings compare to a conventional AHU. In the actual case,

the energy saved is higher than the calculated value due to the following reasons,

a) higher coil surface temperature,

b) longer coil length,

c) higher condensate water outlet temperature,

d) more condensate water collected during rainy days.

For a direct expansion refrigerant type AHU, the compressor cuts in and out

from time to time according to the cooling load. For this type of AHU, the cooling

load profile demonstrates zigzag fluctuating behavior. The condensate water collected

will not achieve 100% as per a modulating chilled water type AHU. Experiments

found that the amount of condensate water collected for this type of AHU is around

65% as compared to a modulating type chilled water AHU. Therefore, the total

energy saved for the direct expansion refrigerant type AHU is 1% 0.01%.

The energy saving of 1% to 2% is attractive in air-conditioning system. In

simple theory, as long as heat is transferred between the cooled condensate water and

the warm fresh air, energy is saved. The operating cost is negligible compare to the

energy saved and the capital cost is negligible compare to the AHU construction cost.


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7.0 REFERENCE

1. Incropera, F.P. and Dewitt, D.P. Fundamentals of heat and mass transfer.

(5 th ed.). John Wiley & Sons. New York, 2002.

2. ASHRAE. (2001). Climate Design Information.

3. Street, R.L., Watters, G.Z. and Vennard, J.K. Elementary Fluid Mechanics.

(7 th ed.). John Wiley & Sons. New York, 1996.

4. Spang, B. Correlations for Convective Heat Transfer. The Chemical

Engineers Resource Page . http://www.cheresources.com/convection.pdf

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