Heat Transfer, Refrigeration and Air Conditioning Questions

• In refrigeration practice, a thermostatic expansion valve (TEV or TXV) is typically matched with which type of evaporator arrangement to control superheat and prevent liquid floodback?
• Two large parallel plates (emissivity = 0.7) exchange radiation. To cut the net radiant heat transfer by 75%, thin parallel shields (emissivity = 1) are inserted. How many radiation shields are required?
• According to Newton’s law of cooling for convection, the heat transfer rate from a hotter body to a colder fluid is proportional to which factors?
• Emissivity is defined as the ratio of the emissive power of a real surface to that of an ideal black body at the same temperature. Is this statement correct?
• In thermal comfort assessment for HVAC design, which index combines the effects of dry-bulb temperature, relative humidity, and air velocity on the human body?
• For a gas to be liquefied by pressure, its temperature must be set in what relation to its critical temperature?
• Which statement best captures the definition of thermal conductivity k for a homogeneous material?
• With the lower refrigeration temperature fixed, which action improves the coefficient of performance (COP) of a refrigeration cycle?
• What does “one tonne of refrigeration” (1 TR) signify in conventional refrigeration units?
• In psychrometrics, cooling air with no change in its specific humidity (humidity ratio) is termed what process?
• Psychrometric definition — choose the correct term. The following ratio is considered at the same temperature and pressure: actual mass of water vapour present in 1 kg of dry air divided by the mass of water vapour that 1 kg of dry air would hold at saturation. What is this quantity called?
• Aircraft environmental control — most used air-cycle system. Which air cooling (reverse Brayton) system is most commonly used on transport-type aircraft for cabin air-conditioning and cooling?
• Domestic refrigerators — which evaporator type is used most commonly? Identify the evaporator construction typically used in household (frost-free) refrigerators.
• Cooling towers and spray ponds — effect of ambient wet-bulb. How does available cooling capacity change as the ambient air wet-bulb temperature decreases?
• Terminology check — “dehydration” versus “dehumidification”. Evaluate the statement: “Dehydration is the process of removing water vapour from the surrounding air.”
• Vapor-compression refrigeration — COP sensitivity. If the evaporator temperature is held constant, how does the coefficient of performance (COP) change as condenser temperature increases?
• Heat conduction through a thick-walled cylinder — correct formula for Q. For a hollow cylinder of length l, inner radius r1, outer radius r2, thermal conductivity k, with inner surface at temperature T1 and outer surface at T2, select the correct steady heat-transfer rate Q expression.
• Dimensionless groups — convection versus conduction. What is the name of the dimensionless number that represents the ratio of convective to conductive heat transfer across a fluid layer?
• Critical radius of insulation — behaviour up to the critical point. Up to the critical radius of insulation on a cylinder, what is the effect of adding insulation on the heat loss to the surroundings?
• Critical thickness (radius) of insulation — basic relation. Evaluate the statement: “The critical thickness of insulation equals the ratio k / h0, where k is the insulation thermal conductivity and h0 is the external convective heat-transfer coefficient.”