Thermal comfort in a room with exposed roof using evaporative cooling in Delhi Krishan Kant, S.C. Mullick * Centre for Energy Studies, Indian Institute of Technology, New Delhi 110016, India Received 9 April 2001; received in revised form 25 October 2001; accepted 8 November 2001 Abstract In this study, diurnal hourly values of temperature and humidity in a room, with its roof exposed to the solar radiation in the summer months of April, May and June in Delhi, and having a direct evaporative cooler, have been computed and compared with the values obtained if the roof is unexposed. It is seen that additional heat load due to exposure of the roof to the solar radiation can be taken care of by slightly increasing the 4ow rate of the evaporatively cooled supply air. For April and May, the resulting conditions of the room air can be brought within an extended comfort zone (ECZ) by selecting an appropriate air change rate (ACH) giving due consideration to the bypass factor (BPF) of the cooler. During the month of June, the ambient air is very hot and its humidity rises; the conditions of the room are not within the ECZ, although the discomfort is mitigated. The levels of thermal sensation, which may be obtained with a direct evaporative cooler, have been computed on a numerical scale (S) which is pertinent to hot-dry and warm-humid climatic zones in India. Keywords: Evaporative cooling; Exposed roof; Summer of Delhi 1. Introduction The main purpose of a building is to provide shelter from the vagaries of nature. The roof protects us from the intense heat from above. It causes a decrement in the amplitude and a time lag between the temperature 4uctuations that are transmitted through it. The hot quartet of April-June is a trying period for the people who live in places such as Delhi and more so for those whose o=ces are located on the top 4oors of the multi-storied buildings. The indoor temperature is higher in case of a room with its roof exposed to the solar radiation as compared to a similar room on a lower 4oor wherein the roof is a partition. Under such conditions, a direct-evaporative cooler offers an inexpensive and eco-friendly way to achieve thermal comfort. It can provide large amounts of cool air to sweep away the room heat loads. The advantages of the evaporative air cooler over the halogen-based air conditioners are beautifully brought out by Mathews et al. [1]. The evaporative cooling process is based on evaporation of water directly into the supply air lowering the sensible heat of the air and water vapour mixture but raising its hu- midity. A suitable combination of air change rate (ACH) and bypass factor (BPF) can be selected to achieve comfort con- ditions or to mitigate discomfort in a room depending on the outdoor temperature and humidity as discussed by the au- thors [2]. The walls of the room (Appendix A) were modeled using the ASHRAE transfer function approach [3] and sim- ulations were carried out with a computer program, TRN- SYS [4], for computing the hourly values of room air tem- perature and humidity. In the present study, the room with its roof exposed to the solar radiation has been considered and compared with a similar room with its roof unexposed. 2. Results and discussions The diurnal hourly values of the room air dry bulb tem- perature (DBT) and relative humidity (RH) in the month of April in Delhi, for an air change rate (ACH = 3 : 5 ac=h) and a bypass factor (BPF = 10%) are plotted in Fig. 1. The