Subject: BRE2031 – Environmental Science
Tutorial 3:Thermal Comfort and Thermal Load Estimation
1. The ventilation heat loss can be determined by: Is this expression used for sensible heat loss or latent heat loss?
2. List 5 parameters that affect thermal comfort and explain how they affect thermal comfort. List and explain 4 types of local thermal discomfort.
3. What is thermal load? Give 3 examples of thermal load.
4. The external wall of a room measures 4.8m by 2.6m and has an average age U-value 1.8W/m2 K. the indoor air temperature is 21oC, the mean radiant temperature is 18 oC, and the outdoor air temperature is 0 oC.
(a) Calculate the environmental temperature inside the room. (19oC)
(b) Use the environmental temperature to calculate the rate of heat loss through the wall. (427W)
5. A house has a floor area of 92 m2 and a ceiling height of 2.5m. The average indoor air temperature is kept at 18 oC, the outdoor air temperature is 6 oC, and the average infiltration rate is 1.5 air changes per hour. The volumetric specific heat capacity of the air is 1300 J/m3K.
(a) Calculate the rate of ventilation heat loss. (1495W)
(b) Calculate the cost of the heat energy lost during 24 hours if the above conditions are maintained and replacement heat cost 7 pence per mega-joule. (904 pence)
6. A sports pavilion has internal dimensions of 11m×4m×3m high. 20 percent of the wall area is glazed and the doors have a total area of 6m2. The U-values in W/m2 K are: walls 1.6, windows 5.5, doors 2.5, roof 1.5, and floor 0.8. The indoor air temperature is maintained at 18 oC when the outdoor air temperature is -2 oC. There are four air changes per hour and the volumetric specific heat capacity of air is 1300 J/m3K. The heat gains total 2200W.
(a) Calculate the net rate of heat loss from this building. (8030W)
(b) Calculate the surface area of the radiators required to maintain the internal temperature under the above conditions. The output of the radiators is 440 W/m2 of radiating surface area. (18.25m2)
7. A room has 7.5m2 area of single-glazed windows, which have a U-value of 5.6W/m2K. It is proposed to double-glaze the windows and reduce the U-value to 3.0 W/m2K. During a 33-week heating season, the average temperature difference across the windows is 7 oC.
(a) For both types of glazing, calculate the total heat loss during the heating season. (5.868GJ; 3.144GJ)
(b) Obtain current figures for the cost of electrical energy and the approximate cost of double glazing windows. Estimate the number of years required for the annual fuel saving to pay for the cost of the double glazing.
8. The average rates of heat loss for a particular house are 1580W total fabric loss and 870W ventilation loss. The seasonal heat gains of the house total 27 500 MJ. The fuel used has a calorific value of 32 MJ/kg and the heating system has an overall efficiency of 75%.
(a) Calculate the input heat required during a heating season of 33 weeks. (21.398GJ) (b) Calculate the mass of fuel required to supply one season’sheating. (892 kg)
Formula：
Indoor environmental temperature tei
Where – tei =indoor environmental temperature (oC); tr =mean radiant temperature (oC); tai =indoor air temperature (oC)
Fabric heat loss
Where – Pf =rate of fabric heat loss (W); U=U-value (W/m2 K) A=area of that element (m2); △t=temp difference (℃)
Ventilation loss
where
Pv =rate of ventilation heat loss (W);cv =volumetric specific heat capacity of air (J/m3 K)
N=air change rate (number of complete air changes per hour) V=volume of the room (m3);△t=temp difference (oC)