EFFECT OF SATURATION EFFICIENCY OF DIRECT EVAPORATIVE COOLER ON INDOOR CONDITIONS AND THERMAL COMFORT IN A SMALL OFFICE ROOM OF A BUILDING DA Hindoliya and SC Mullick * Centre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas New Delhi 11001 (India) ABSTRACT Thermal comfort of occupants and indoor air relative humidity () in an office have been assessed through simulation study. In order to maintain room relative humidity below 5%, the requirement of cooler saturation efficiency and air change rate have been determined for some practical situations. It is observed that, it is not always desirable to have cooler with higher saturation efficiency for applications where low indoor air relative humidity is of primary concern. For such applications, an ordinary evaporative cooler having lower saturation efficiency (0%) may be used by allowing slightly higher air change rate inside the occupied space. INDEX TERMS Indoor air Quality, Thermal Comfort, Evaporative Cooling, Thermal sensation. INTRODUCTION Evaporative cooling can be used to provide thermal comfort in buildings along with greater fresh air and reasonably good indoor air quality. Most direct evaporative coolers draw outdoor air through the wetting media (pads), which trap significant amount of dust and other particles. Also the use of all fresh air reduces CO 2 concentration, odors and volatile organic compounds released from building materials etc. in the occupied space. Since, it is economical, pollution free and simple, there is need to review the possibilities of using direct evaporative cooling for maintaining thermal comfort inside the buildings. Several researchers have made attempts in this direction. Methews et al.(1994) have brought out the possibilities of achieving thermal comfort using direct / regenerative evaporative cooling in well designed buildings in various parts of South Africa. Nation (1984) has carried out similar study for some cities in the U.S.. He has shown that it could save from 10 to 100 % of conventional mechanical cooling. Krishan Kant et al. (2001) have performed a simulation study for thermal comfort in a building at Delhi (India) using direct evaporative cooling. They found that by proper combination of air change rate (ACH) and bypass factor of cooler, the desired room conditions can be obtained. In India, direct evaporative coolers are widely used in the northern region during the dry months of April and May. In the month of June, humidity of outdoor air increases, the use of such coolers do not produce the satisfactory results. The commonly used coolers utilize aspen or wood fiber pads which have the saturation efficiency of around 0-5%. On the other hand, coolers which utilize cellulose pads have higher saturation efficiency (above 80%). Such coolers are quite expensive and may not be within the reach of common man. However, such coolers may result in higher indoor air relative humidity which some times may not be suitable for buildings such as office having electronic appliances like personal computers, Xerox / Fax machines and other sophisticated appliances. In the present work, performance of above two types of evaporative coolers have been compared through simulation study. An attempt has been made to assess the possibility of obtaining thermal comfort of occupants along with maintaining indoor air relative humidity below 0%. Diurnal hourly values of room temperature and humidity have been calculated assuming cooler saturation efficiency of 0% and 80% for summer months of April, May and June in Delhi. THE ROOM SPECIFICATIONS AND SIMULATION PARAMETERS A small office building (having size 4m x 4m x 3.m) coupled with a direct evaporative cooler has been simulated. The room is having its south wall exposed to solar radiation. Other walls, floor and ceiling are considered as interior partitions. The heat gain from office equipments and other appliances has been considered (Table 1) as * Corresponding author email: Subhash_cmullick@yahoo.com 254
suggested in ASHRAE handbook of fundamental (2001). Exterior wall consists of 100 mm common brick wall with 25 mm stucco and 20 mm plaster. The wall has an overall heat loss coefficient of 2. W/m 2 -K, which has been selected from TRNSYS (2000). The room is occupied by 3 persons doing light work (activity level 5). Capacitance of room air and furnishing is 1000 kj/k. The room has been modeled using transfer function approach as per ASHRAE handbook of fundamental (1997) using TRNSYS. Weather data has been obtained from hand book of Mani (1981). Table 1. Assumed Internal Heat gain from lighting and Appliances in a smalloffice Room S.No. Lighting/Appliance Heat Gain Nos. Total Convective Radiative 1 Lighting - - 53 37 10 2 Computers 55 2 110 99 11 3 Monitors 70 2 140 91 49 4 Fax Machine 10 1 10 7 3 5 Laser Printer 10 1 10 144 1 Photocopier 400 1 400 320 80 7 Ceiling Fan 70*0.35 1 24.5.25.25 Total Gain 1380.5 1049.25 331.25 Table 2. Comparison of Average Values of Room Temperatures and Relative Humidity with Two Different Direct Evaporative Coolers 80% 0% Month Room Condition Saturation Saturation ACH (ac/h) April Temperature ( 0 C) Relative Humidity (%) 28.04 55.5 29.94 4. ACH (ac/h) 20 20 May Temperature ( 0 C) 27.54 29.85 Relative Humidity (%) 1.75 50.30 ACH (ac/h) 27 27 June Temperature ( 0 C) 29.29 31.28 Relative Humidity (%) 9.4 59.04 RESULTS AND DISCUSSION Figure 1 depicts hourly room conditions for a small office room for the month of April in Delhi. The room conditions have been analyzed with respect to tropical summer index (TSI) lines drawn on psychrometric diagram. TSI has been developed by Sharma and Ali (198). The region bound between 25 and 30 0 C TSI lines (shown by firm lines) represents comfort region which corresponds to indoor air velocity of 0 m/s and the region bound between 25 and 30 0 C TSI lines (shown by chain lines) represents comfort region which corresponds to indoor air velocity of 2.5 m/s. The curves indicate hourly variation of room dry bulb temperature and specific humidity if cooler having saturation efficiency of 80% is used with air change per hour. The resulting room conditions are comfortable with room air relative humidity below 0% as indicated by left curve. The right curve indicates the resulting room conditions with cooler saturation efficiency of 0%. Figure 2 shows the thermal sensation of occupants on a numerical scale. Any thermal sensation number lying between two dotted lines are considered to be comfortable as discussed by Sharma and Ali (198). 255
ac/h with 80% Saturation ac/h with 0% saturation TSI=30 70% 50% 0.0220 0.0200 TSI=25 0.00 V=0 m/s 24 Ambient 0.010 0.0140 0.00 0.0100 0.0080 0.000 20 22 24 2 28 30 32 34 3 38 40 Dry-Bulb Temperature ( 0 C) V=2.5 m/s 30% 20% Sp.Humidity ( kg/kg) Figure 1. Diurnal Variation of Room Temperature with ac/h for April in New Delhi ac/h with 80% Saturation and Air Velocity of 0 m/s ac/h with 0% Saturation and Air Velocity of 1 m/s Thermal Sensation, S 5 4 Comfortable 3 2 1 2 3 4 5 7 8 9 10 11 13 14 15 1 17 19 20 21 22 23 24 Time (Hours) Figure 2. Thermal Sensation of Occupants with ac/h for April in New Delhi Similar results for the month of May are presented in Figs. 3-4. In this month, almost similar room conditions as that of April, can be produced by having 20 air change per hour. In June, (Figs.5-), room conditions would be comfortable with 27 air change per hour. However the room air relative humidity is higher compared to that obtained in April and May. In addition, indoor air velocity would be required which may be obtained by using a ceiling fan. The average values of room temperature and relative humidity with two different direct 25
evaporative coolers are also presented in Table1. 20 ac/h with 80% Saturation 20 ac/h with 0% Saturation TSI=30 70% 50% 0.0220 0.0200 TSI=25 0.00 V=0 m/s 24 V=2.5 m/s Ambient 30% 20% 0.010 0.0140 0.00 0.0100 0.0080 Sp.Humidity ( kg/kg) 0.000 20 22 24 2 28 30 32 34 3 38 40 Dry-Bulb Temperature ( 0 C) Figure 3. Diurnal Variation of Room Temperature with 20 ac/h for the Month of May 20 ac/h with 80% Saturation and air Velocity of 0 m/s 20 ac/h with 0% Satura0tion and air Velocity of 1 m/s Thermal Sensation, S 5 4 3 Comfortable 2 1 2 3 4 5 7 8 9 10 11 13 14 15 1 17 19 20 21 22 23 24 Time (Hours) Figure 4. Thermal Sensation of Occupants with 20 ac/h for the Month of June 257
27 ac/h with 80% Saturation 27 ac/h with 0% Saturation 70% 50% 0.0220 0.0200 0.00 V=0 m/s V=2.5 m/s 24 Ambient 30% 0.010 0.0140 0.00 Sp.Humidity ( kg/kg) 20% 0.0100 TSI=25 TSI=30 0.0080 0.000 20 22 24 2 28 30 32 34 3 38 40 Dry-Bulb Temperature ( 0 C) Figure 5. Diurnal Variation of Room Temperature with 27 ac/h for the Month of June 27 ac/h with 80% Saturation and air velocity of 1 m/s 27 ac/h with 0% Saturation and Air Velocity of 2.5 m/s Thermal Sensation, S 5 4 3 Comfortable 2 1 2 3 4 5 7 8 9 10 11 13 14 15 1 17 19 20 21 22 23 24 Time (Hours) Figure. Thermal Sensation of Occupants with 27 ac/h for the Month of June CONCLUSIONS Evaporative cooling can be used successfully to maintain thermal comfort of occupants by having an appropriate air change rate depending on the out door conditions. For Applications where low relative humidity has to be maintained the cooler having low (around 0%) saturation efficiency may be used, however, with such cooler some what higher air change rate would be needed. The present work reveals that the direct evaporative cooler 258
with high saturation efficiency may not be desirable at the locations where outdoor air conditions are not like desert and for applications where low indoor air relative humidity is required. REFERENCES Mathews EH., Klein geld M. and Grober LJ. Integrated (1994), Simulation of Buildings and Evaporative Cooling System, Building and Environment 29,2, 197-20. Nation JA. (1984), Evaporative Cooling in Nontraditional Climates, ASHRAE Trans. 90, 154-15. Krishna K., Kumar A. and Mullick SC.(2001), Space Conditioning Using Evaporative Cooling for Summer in Delhi, Building and Environment, 3,1, 15-25. ASHRAE handbook of Fundamentals (2001), American Society of Heating, Refrigeration and Air Conditioning Engineers, Inc., 1791 Tullie Circle N.E. Atlanta. TRNSYS: a transient system simulation program, version 15, (2000) Solar Energy Laboratory, University of Wiscosin, Medison. ASHRAE handbook(1997)- Fundamentals, American Society of Heating, Refrigeration and Air Conditioning Engineers, Inc., 1791 Tullie Circle N.E. Atlanta. Mani A.(1981), Hand book of Solar Radiation Data for India, Allied Publishers Pvt. Ltd. New Delhi. Sharma MR. and Ali S. (198), Tropical Summer Index a study of Thermal Comfort in Indian Subjects. Building and Environment, 21(1), 11-24. 259