(a) A building in the Midlands has an air conditioning system that uses re- circulation of 'all air' in order to reduce energy consumption and hence running costs. The total mass flow rate of air is 8 kg/s with a minimum fresh air fraction of 30%. The hall is to be maintained at a constant enthalpy of 0.04MJ/kg, and it can be assumed that the off-coil enthalpy will be kept constant at 0.03MJ/kg. The maximum coil duty is 0.12MW. The dampers move to minimum fresh air when the outdoor enthalpy exceeds the room enthalpy. Use the data below to calculate the expected energy consumption for a chiller with a constant COP of 2.8. Outdoor Enthalpy (MJ/kg) Annual Frequency of Occurrence (%) 0.01 -0.02 13 0.02 -0.03 23 0.03-0.04 22 0.04 -0.05 14 0.05 -0.06 11 > 0.06 4 (15 marks) (b) Describe the energy consuming processes within an air-conditioning system, and explain why there is an energy penalty for close control of indoor space conditions. (10 marks) Question 1: (a) Sketch system and psychrometrics hm = mixed enthalpy, ho = outdoor enthalpy, hc = off-coil enthalpy when hm hr use minimum fresh air. I.e. when ho > 40 |hm = 0.3 h + 0.7 x 40 hm-hc 0.3ho+28-30 0.3ho-2 At full load hm max = 30+(120/8) = 45 (45-0.7x40)/0.3 = 56 ho max so when ho 55 |hm-hc = 45-30 = 15 2 1 1 1 2117 ho ho bar f hm-he f(hm-hc) 30-40 35 22 5 110.0 40-50 45 14 11.5 161.0 50-60 55 11 14.5 159.5 >60 60 4 15 60.0 490.5 3 F=8x575x8760/(3x100) 114580.8 kWh 21 15 (b) Sketch of typical all-air system Pre-heat and frost protectection. 21 ead aloud - jum max ho max = (45-0.7x40)/0.3 = 56 so when ho=55 |hm-hc = 45-30 = 15 + 8 of 12 (D 1 1 1 ho ho bar f hm-hc f(hm-hc) 30-40 35 22 5 110.0 40-50 45 14 11.5 161.0 50-60 55 11 14.5 159.5 >60 60 4 15 60.0 490.5 3 F = 8x575x8760/(3x100) 114580.8 kWh 2 15 (b) Sketch of typical all-air system Pre-heat and frost protectection. Humidification. May require extra preheating if using water spray Steam humidification - cost of raising steam. Cooling and dehumidification Dehumidification may require re-heat for close humidity control Support with psychrometric skethes 2 1 1 1 1 1 3 10