ARE346P HVAC Design Exam April 22, 2010

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Problem:

1 ______/ 75

2 ______/ 50

3 ______/ 125

Total ______/ 250

SHOW ALL OF YOUR WORK AND CLEARLY IDENTIFY POINTS AND PROCESSES IN THE ATTACHED PSYCHROMETRIC CHARTS.

There are several psychometric charts and the p-h diagram and table for ammonia at the end of the exam. Problem 1

For the ductwork in the figure below: the length of duct ‘1’ is 30m, duct ‘2’ 20m, and duct ‘3’ 60m. In both rooms there is a diffuser with the pressure drop of 20 Pa. AHU connection 1 2 Fitting CD3-10 r/D = 2.5 (Table 18.7) SD5-10 (Table 18.8) 3

V3 = 1500 l/s, Pto V2 = 3500 l/s, Pto

Q = 2000 L/s, Ptot = 200 Pa Q = 2000 L/s, Ptot = 200 Pa t = a) 55 points t = 200 Size the ductwork in the figure below using equal friction method. Friction loss should be in the range from 1 to 2 Pa/mlengt. Calculate total pressure drop and balance the system. Use round ducts. b) 10 points If the duct space is limited to plenum space with height of 0.5m select the square ducts for the same systems (assume that there is no change in local pressure coefficient: elbow and T connection).

c) 10 points If the flow doubles and duct stays the same how much will increase the pressure drop in the system? Draw this process in pressure-volume flow rate diagram. How much will this affect the fan power?

Problem 2 a) 30 points You are using ammonia (use the attached diagram) in the vapor compression freezer system (Figure below). The evaporation is at -20°C, and the condenser is cooled by air at 30°F. Assume that the refrigerant condensation temperature is 10°C higher than air temperature. There is a superheating in the evaporator to temperature of -10°C (T1=-10C).

Calculate the COP of chiller (provide all the readout points and the whole calculation process).

b) 15 points Calculate the COP if the same chiller is cooled by lake water at 25°C. Assume that the refrigerant condensation temperature is 5°C higher than water temperature.

c) 5 points Explain why we assume lower temperature difference between lake water and refrigerant than between air and refrigerant in the condensators of these two systems. Problem 3 a) 20 points Consider the process in the AHU presented in figure below which precisely controls the relative humidity:

Automatic system controls the following parameters: - outdoor air (OA) is at 95°F and 60% RH, - room air (RA) is at 75°F and 50% RH, - the supply air flow rate is 10,000 CFM and 75% is recirculated, - average cooling coil temperature is 45°F, - humidity ratio after the cooling coil is 0.007 lbw/lbda, - temperature after the heater is 55°F Assume constant air density and specific heat.

Draw the schematic in the psychrometric chart (mark all points and processes). b) 20 points How much cooling and heating energy is need for this AHU? c) 30 points Consider the process in the modified AHU presented in figure below where the “dual heat exchanger system” is used instead of the heater. This system precisely controls the relative humidity and to avoid to use reheater a dual heat exchanger system is used (heat exchangers 1 and 2 and the pump system that circulates water in- between them).

Average cooling coil temperature is the same (45°F) like in the previous AHU’s configuration as well as OA, RA. Also, air temperature before the fan (RH or temperature) is 55°F, and the humidity ratio after the cooling coil is 0.007 lbw/lbda,

For the “dual heat exchanger system”: water entering the heat exchanger 1 is 63°F and water entering the heat exchanger 2 is 67°F (assume no heat losses in the pipe and neglect change of the temperature in the pipe system due to the pump operation).

Draw the process in the psychometric chart. d) 15 points For the AHU in part (c) calculate how much electric power is needed for the pump if the pump’s total efficiency (including efficiency of the motor) is 75%. The total pipe pressure loss is 2ft including pressure drop in both heat exchangers. e) 20 points Considering the two AHUs (from parts a and c) and show which system is more efficient and why (show the energy consumption comparison for considered design conditions).

f) 20 points What is the effectivness of the heat exchanger 1 (in AHU of case c))? What is the UoAo value for this heat exchanger? (Assume F=0.85)