Energy and Buildings 42 (2010) 822–827

Experimental investigation of dehumidification process in cooling coil by utilizingair-to-air heat exchanger in humid climate of Iran

Shahram Delfani a,*, Hadi Pasdarshahri b, Maryam Karami a

a Department of Installation, Building and Housing Research Center (BHRC), PO Box 13145-1696, Tehran, Iranb Department of Mechanical Engineering, Tarbiat Modares University, PO Box 14115-143, Tehran, Iran

A R T I C L E I N F O

Article history:

Received 23 September 2009

Received in revised form 7 December 2009

Accepted 10 December 2009

Keywords:

Air-to-Air heat exchanger

Heat recovery

Cooling coil

Humid area

Experimental study

Iran

A B S T R A C T

Energy recovery systems can be used in HVAC to reduce energy consumption by recovering wasted

energy from the exhausted air to pre-cool inlet fresh air. In this paper, experimental study of utilizing

cooling coil (CC) and cooling coil combined with heat exchanger (CCHE) has been developed. Considering

the different climate zones in Iran, the experimental study has been done for the areas with high

humidity content. In each case, the effects of air-to-air heat exchanger (HE) on the sensible and latent

cooling load are investigated. The analysis gives the applicability of HE in Iran for various operational

conditions to obtain air with lower moisture content. In CCHE systems more latent load quota is

allocated in cooling coils and lower air humidity ratio can be obtained. Also in the cities with higher dew

point depression (difference between dry-bulb and dew point temperature) more humidity reduction is

achieved and the difference between latent load quotas of CCHE system is higher than CC system.

� 2009 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Energy and Buildings

journa l homepage: www.e lsev ier .com/ locate /enbui ld

1. Introduction

Energy consumption nowadays is an important issue in theworld and also in Iran. Within two past decades, the rate of theenergy consumption of buildings has increased due to urbaniza-tion and industrialization, which necessitates energy savingstrategies. There are many strategies that can be applied to reduceenergy consumption especially in heating and cooling devices. Oneof the strategies is applying energy recovery systems whichrecover waste energy from buildings and other ventilated spaces.Generally, air recovery systems contain HE. HEs are usuallydesigned in cross flow compact form pattern. In heating andcooling devices the indoor air passes through one side and fresh airpasses through the other side. The energy exchanges between twostages, therefore, the fresh air gets cooled or heated depending onindoor air temperature. In summer, energy recoveries are appliedto pre-cool inlet fresh air and reduce energy consumption ofcooling or air-conditioning system.

In order to evaluating the performances of HE systems,Lazzarin and Gasparella [1] developed economical analysis toshow the benefits of using HE systems even for small ventilationflow rate. Fehrm et al. [2] presented a review about developing theheat recovery techniques and their economics in Germany andSweden. A theoretical estimation of the performance of themembrane base ventilator by analyzing heat and mass process is

* Corresponding author. Tel.: +98 21 88255942; fax: +98 21 88255941.

E-mail address: delfani@bhrc.ac.ir (S. Delfani).

0378-7788/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.enbuild.2009.12.002

developed by Zhang and Niu [3] and Zhang et al. [4]. Roulet et al.[5] analyzed theoretically the real energy with air handling unitsand gave some offers about heat recovery choices withexperimental point of view. Ke and Yanming [6] investigatedheat exchangers for the energy saving in buildings with littlemoisture emission. Their analysis gave the applicability of HE inChina. Rose et al. [7] carried out a quasi-steady-state model of acounter-flow HE by taking into account the effects of condensa-tion and frost formation. The model is used to determine thecontrol strategy for heat exchangers under northern European andarctic climate conditions. Joudis [8] express the importance ofenergy recovery equipments with analyzing the heat transferthrough building shells. The author showed that the heat transferthrough the building shells is less than energy consumptionresulted by ventilation. The other important aspect of the HE is thetrade-off between reduction of energy consumption and equip-ment cost. Dieckmann et al. [9] indicated that the popularizationrate of energy recovery ventilator reaches only 1% in the UnitedStates due to some reasons including the equipment price andmaintenance. The performance of energy recovery ventilatorunder various climate conditions and different indoor set pointtemperatures is simulated by energy simulation environmentprogram, Energy Plus [10], by Zhou et al. [11]. The results showthat the heating recovery in summer plays a more significant rolequantitatively than cooling recovery in winter.

In this research an experimental investigation of HE to reduceenergy consumption in buildings and also cooling coil airdehumidification is developed. The investigation is done basedon different outdoor design conditions of four cities in Iran in

Nomenclature

DBT dry-bulb temperature (8C)

DPD dew point depression (8C)

DPT dew point temperature (8C)

HRD humidity ratio difference (kgdry air kgwater�1)

HRR humidity ratio reduction (kgdry air kgwater�1)

LLD latent load difference (kJ kgdry air�1)

Qcoil cooling coil load (kJ kgdry air�1)

Qlatent latent load (kJ kgdry air�1)

Qsensible sensible load (kJ kgdry air�1)

SLD sensible load difference (kJ kgdry air�1)

WBT wet-bulb temperature (8C)

S. Delfani et al. / Energy and Buildings 42 (2010) 822–827 823

humid climate area. The experimental procedures contain twoparts including CC and HE combined with CC for dehumidification.

2. Experimental apparatus and procedure

To investigate the different cycles and prepare various outdoorconditions for experimental analysis, an experimental setup isprovided. Fig. 1a, shows the schematic diagram of experimentalsetup. It contains a blower, air simulator, air-to-air heat exchanger,cooling coil and measuring instruments.

The body of air simulator including a centrifugal fan foradjusting flow rate is stainless steel. Five heaters are installedinside the simulator to prepare required heat and set pointtemperature for incoming air. These heaters totally can provide a

Fig. 1. Schematic of experimental setup. (a) Air-to-air heat e

wide range of power to control air temperature. In the lower ofsimulator body a water tank is located with the same heater. Thissection is used to control air humidity by injecting water vapor.With the two described parts together, air temperature andhumidity can be controlled separately. A straighter is provided toachieve airflow uniformity and a free rotational mixer downstreamof simulator to obtain temperature and relative humidityhomogeneity. Temperature, humidity and velocity are measuredseparately in each state point on experimental setup. Two hot-bulbsensors which are located in the airflow channel are used tomeasure air velocity based on ASTM-3461 [12]. Also, twotemperature and two humidity sensors are situated in the inletand outlet of HE and cooling coils. A cross flow HE is used as heatrecovery. It is covered by an isolated duct. Each part of the HE andducts are sealed against leakage before experiment.

The accuracy and resolution in measuring temperature arewithin �0.3 8C and 0.01 8C respectively. The relative humidity ismeasured by the sensors in the range of 10–95% and the accuracy andresolution are �3% and 0.01% respectively. Also, average air velocityis measured using thermal anemometers with accuracy of reading�4% and resolution 0.01 m/s. The experimental apparatus is shownin Fig. 1b.

3. Test procedures

Due to seas, deserts, mountains and jungles in Iran, wide rangeof climates and different seasons have been seen in this country. Inother words, Iran is a multi-climate country. Each country can bedivided into different climatic areas depending on minimum andmaximum values of temperature and relative humidity which canbe found by weather statistical data. Various climates in Iran areshown in Fig. 2. Based on outdoor design conditions in summer,

xchanger with cooling coil. (b) Experimental apparatus.

Table 1Selected cities for experimental analysis [13].

City Sea level (m) Longitude Latitude DBT (8C) WBT (8C) DPD (8C)

Abadan 7 488150 308220 47.4 29.4 22.72

Booshehr 20 508510 288590 40.8 30.9 12.16

Chabahar 25 608370 258170 36.3 30.9 6.6

Ramsar �20 508400 368540 30.2 26.4 4.87

Fig. 2. Different weather regions based on outdoor design conditions.

S. Delfani et al. / Energy and Buildings 42 (2010) 822–827824

there are five different regions in Iran (Hot & Dry, Hot & Humid, Hot& Semi-Humid, Moderate & Dry and Moderate & Humid). Actually,in humid areas to prepare suitable air condition for variousprocesses such as air-conditioning, the outdoor air should bedehumidified. Generally, in these areas the outdoor air isdehumidified using cooling coil. The considerable ratio of coolingload is shared for dehumidification to prepare latent load.Therefore, four cities are selected for comprehensive study andtheir weather conditions. These cities are located in Moderate &Humid and Hot & Humid regions. Table 1 shows the selected cities.

As shown in Fig. 1a, outdoor design conditions for the selectedcities of Table 1 are simulated in the laboratory in state point 1 byusing of air simulator. The experiments are carried out in twocases. In first case, only cooling coil is used for dehumidification.Also, air temperature and humidity are measured before and aftercooling coil and recorded respectively. In second case, the testsetup is carried out to examine cooling coil with recovery. Hence,heat recovery effect in humidity reduction value and energyrecovery can be investigated. Each experiment is conducted in arange of 30–60 min when a steady state is reached for a given

test condition. Dry-bulb temperatures and relative humilitiesare measured when all measured data showed negligiblevariation. The air volume rate in all experiments was kept constantat 34 m3/min.

4. Results and discussion

In general, the amount of latent heat value versus sensible heatis higher in hot and humid cities such as Booshehr and Abadan inIran. In these cities, to achieve comfort conditions or obtain dry airfor some processes, ambient air humidity should be reduced.When air cools lower than its dew point temperature, its humiditycontent begins to condensate.

The mentioned two test cases for each city are shown in Fig. 3on Psychometric chart for the selected cities. In all experimentalcases the cooling rate in cooling coil is assumed constant(Qcoil = 55 � 5 kJ kgdry air

�1). With regarding of this figure, it can beseen that by use of HE, dehumidification rate increases and at the endof process, lower air humidity is available. Therefore, the more dry aircan be obtained at the end of CCHE system. For further evaluation, the

Fig. 3. The Psychometric process of each experiment.

S. Delfani et al. / Energy and Buildings 42 (2010) 822–827 825

dew point depression (the difference between dry-bulb and dewpoint temperature) is defined by Eq. (1):

DPD ¼ DBT � DPT (1)

The value of DPD for each city is shown in Table 1. In the selectedcities, DPDAbadan > DPDBooshehr > DPDChabahar > DPDRamsar. It can beseen that in Abadan city due to higher DPD, no dehumidification isoccurred in HE (process A1–A2). On the other hand, in Booshehr,Chabahar and Ramsar dehumidification can be achieved in HE. Dueto lower DPD, the air temperature reduced to less than DPT bypassing through heat exchanger and the humidity content iscondensed. In order to design CCHE in such cities, appropriatedevices should be utilized in HE to collect condense water.

Fig. 4. Variation of latent heat for each city

While using of HE, first outdoor air temperature with passingthrough HE reduced sensibly (state point 2 for each cities). Then,air temperature and humidity decrease with passing throughcooling coil and finally increase with passing through HE. Thisprocess can change sensible and latent load values in cooling coil.

Since the aim of experiment is air dehumidification, sensibleand latent load quota is studied during each test case. Fig. 4illustrates the values of latent load in cooling coil for CC and CCHEsystems. In all cities, latent heat amount is higher in CCHE systemcompared to CC. In CC system all the cooling load includingsensible and latent are prepared by cooling coil but it can beobserved that some extent of sensible load required for tempera-ture decline to dew point, is provided by HE in CCHE system. Thus,

under different experimental setups.

Fig. 5. Variation of sensible heat for each city under different experimental setups.

S. Delfani et al. / Energy and Buildings 42 (2010) 822–827826

higher amount of total load, in cooling coil is allocated to latent andhumidity ratio reduction.

The latent load difference is defined by Eq. (2):

LLD ¼ Q latent;CCHE � Q latent;CC (2)

Based on Eq. (2) and Fig. 4, for Abadan city the value of LLD is higherthan the other cities. Mainly, the LLD difference in each city iscaused by the variation of DPD. For the cities with higher DPD, theLLD increased. Sensible heat exchangers are more advantageous inthe cities with higher DPD because more sensible load can bereduced in such cities and the amount of latent load in cooling coilis raised considerably.

Fig. 5 shows the amount of sensible load in each experimentaltest for CC and CCHE systems. In CCHE systems some part ofsensible load decreased by utilizing HE, therefore, the amount ofsensible load in cooling coil is lower in CCHE system compared to

Fig. 6. Percentage of latent and se

CC system. The sensible load difference (SLD) is defined by Eq. (3):

SLD ¼ Q sensible;CC � Q latent;CCHE (3)

SLD value is higher in Abadan than the other cities. As discussedbefore, Abadan has higher DPD and most of the sensible load ofoutdoor air is reduced by heat exchanger; therefore, lower sensibleload quota is reduced in cooling coil. Cities with higher DPD havemore SLD because of the efficient use of heat exchanger.

In order to compare CCHE system with CC system thepercentage of each load including sensible and latent for eachcity is illustrated in Fig. 6. In all cases, CCHE systems include morelatent load in cooling coil than CC systems. To emphasize on thepercentage of variation in each city the line slope of latent load ofeach case is depicted in Fig. 6. It can be concluded that for the citieswith higher DPD this slope increases.

Fig. 7 presents air humidity ratio reduction (HRR) in each testcase. The HRR value in CCHE system is lower than CC system in all

nsible heat in each condition.

Fig. 7. Variation of humidity ratio reduction for each city under different experimental setups.

S. Delfani et al. / Energy and Buildings 42 (2010) 822–827 827

cities. Therefore, for the processes where less air humidity isrequired, a CCHE system is more beneficial than CC system. HRR

value is affected by latent load portion in cooling load. Because moreload portion are shared for latent load in CCHE system, the resultingoutlet air has lower humidity ratio compared to CC system. Thehumidity ratio reduction difference (HRD) is defined by Eq. (4):

HRD ¼ HRRCCHE � HRRCC (4)

HRD is higher in Abadan than the other cities because of higherDPD.

5. Conclusion

The objectives of this study were to evaluate the effects of HE onhumidity reduction of CC in humid areas. In all experiments, CCload and air mass volume are assumed constant. The resultsindicate that LLD and HRD in CCHE system are higher than CCsystem. In other words, SLD value in CC system is more than CCHEsystem. Actually, in CCHE system higher amount of total load, incooling coil is allocated to latent and humidity ratio reduction. Theexperimental results show that DPD is the main factor which hasdirect influence on the LLD, SLD and HRD. Mainly, the value of LLD,SLD and HRD increases by DPD. The use of HE has more benefit inthe cities with higher DPD because more sensible load can bereduced in such cities and the amount of latent load in cooling coilis increased considerably.

References

[1] R.M. Lazzarin, A. Gasparella, Technical and economical analysis of heat recovery inbuilding ventilation systems, Applied Thermal Engineering 18 (1988) 47–67.

[2] M. Fehrm, W. Reiners, M. Ungemach, Exhaust air heat recovery in buildings,International Journal of Refrigeration 25 (4) (2002) 439–449.

[3] L.Z. Zhang, J.L. Niu, Energy requirements for conditioning fresh air and the long-term savings with a membrane-based energy recovery ventilator in Hong Kong,Energy 26 (2) (2001) 119–135.

[4] Y. Zhang, Y. Jiang, L.Z. Zhang, Y. Deng, Z. Jin, Analysis of thermal performance andenergy savings of membrane based heat recovery ventilator, Energy 25 (2000)515–527.

[5] C.A. Roulet, F.D. Heidt, F. Foradini, M.C. Pibiri, Real heat recovery with air handlingunits, Energy and Buildings 33 (2001) 495–502.

[6] Z. Ke, K. Yanming, Applicability of air-to-air heat recovery ventilators in China,Applied Thermal Engineering 29 (2009) 830–840.

[7] J. Rose, T.R. Nielsen, J. Kragh, S. Svendsen, Quasi-steady-state model of a counter-flow air-to-air heat-exchanger with phase change, Applied Energy 85 (2008) 312–325.

[8] E. Juodis, Extracted ventilation air heat recovery efficiency as a function ofbuilding’s thermal properties, Energy and Buildings 38 (6) (2006) 568–573.

[9] J. Dieckmann, K.W. Roth, J. Brodrick, Air-to-air energy recovery in buildings,ASHRAE Journal 45 (8) (2003) 57–58.

[10] D.B. Crawley, L.K. Lawrie, F.C. Winkelmann, W.F. Buhl, et al., Energy-Plus:creating a new generation building program, Energy and Buildings 33 (4)(2001) 319–331.

[11] Y.P. Zhou, J.Y. Wu, R.Z. Wang, Performance of energy recovery ventilator withvarious weather temperature set-points, Energy and Buildings 39 (2007) 1202–1210.

[12] ASTM D3464-96, Standard test method for average velocity in a duct using athermal anemometer, 2007.

[13] G. Heidarinejad, M. Heidarinejad, S. Delfani, Outdoor design conditions data forthe cities of Iran, in: AIAA 6th International Energy Conversion EngineeringConference (IECEC), Cleveland, OH, July 28–30, 2008.