Question 1

Psychrometrics: sensible heat factor for cooling and dehumidification  For a process over a cooling coil, with h1 = enthalpy of entering air, h2 = enthalpy of leaving air, and hA = enthalpy at the end of dehumidification (apparatus dew point), choos…

Accepted Answer

Given data h1: enthalpy of entering air h2: enthalpy of leaving air hA: enthalpy at apparatus dew-point (end of dehumidification line) Concept/Approach For cooling and dehumidification, the sensible heat factor is the ratio of sensible heat to total heat for the 1→2 process and, in terms of psychrometric geometry, is given by the segment ratio along the process line. Derivation (sketch-based reasoning) Total heat removed = (h1 − h2); sensible component corresponds to (h1 − hA). SHF = (h1 − hA) / (h1 − h2). Sanity check If there is no dehumidification, hA → h2 and SHF → 1, as expected for purely sensible cooling. Final Answer SHF = (h1 − hA) / (h1 − h2)

Question 2

Human thermal comfort: seasonal change in optimum effective temperature  Compare the optimum effective temperature for human comfort in winter versus summer. Choose the correct relation.

Accepted Answer

Concept/Approach The effective temperature concept accounts for dry-bulb temperature, humidity, and air motion to reflect perceived comfort. Seasonal clothing and acclimatization shift the comfort zone. Reasoning In winter, lower air movement and heavier clothing lead to a slightly higher optimum effective temperature compared to summer, where higher air movement and lighter clothing make a lower effective temperature feel comfortable. Final Answer Higher in winter than in summer

Question 3

Refrigeration maintenance: defrosting by compressor shutdown  Is defrosting of a domestic refrigerator achievable by stopping the compressor for a short period (off-cycle defrost)? Choose the correct truth value.

Accepted Answer

Concept/Approach When the compressor is stopped, coil temperature rises above the frost point, allowing accumulated frost to melt (off-cycle defrost), especially in medium/high evaporating temperature systems. Notes Other methods include electric heaters and hot-gas defrost for low-temperature (freezer) systems, but stopping the compressor is a standard defrost option in suitable applications. Final Answer True

Question 4

Condensing temperature comparison: water-cooled versus air-cooled condenser  For similar load and ambient conditions, a water-cooled condenser typically operates at what relative condensing temperature compared to an air-cooled condenser? Choose the…

Accepted Answer

Concept/Approach Water has higher heat-transfer coefficients and can be supplied cooler than ambient air (via cooling towers or mains), so the condenser saturation temperature (and pressure) can be reduced. Implication Lower condensing temperature reduces compressor work and improves system COP. Final Answer Lower condensing temperature

Question 5

Material selection: condenser tubes for ammonia refrigeration  For ammonia (NH3) refrigerating systems using shell-and-tube condensers, which tube material is appropriate considering compatibility and corrosion? Choose the correct material.

Accepted Answer

Concept/Approach Anhydrous ammonia is incompatible with copper and copper alloys (e.g., brass), which can suffer corrosion/stress corrosion cracking in ammonia service. Therefore Steel tubes are used for condensers/evaporators in ammonia systems.

Question 6

Psychrometrics numeric: compute sensible heat factor from sensible and latent rates  A process adds sensible heat = 30 kJ/s and latent heat = 20 kJ/s to moist air. Calculate the sensible heat factor (SHF) for the process. Choose the correct value.

Accepted Answer

Given data Sensible heat rate = 30 kJ/s Latent heat rate = 20 kJ/s Concept/Approach Sensible heat factor: SHF = (Sensible heat) / (Total heat) = S / (S + L). Step-by-step calculation S + L = 30 + 20 = 50 kJ/sSHF = 30 / 50 = 0.60 Final Answer 0.6

Question 7

Conduction comparison across geometries: thick sphere versus thick cylinder  Assess the statement: Heat transfer by conduction through a thick spherical wall is the same as through a thick cylindrical wall. Choose the correct truth value.

Accepted Answer

Concept/Approach In steady radial conduction, the governing area for heat flow varies with radius: Acyl = 2πrL (cylinder) versus Asph = 4πr² (sphere). This leads to different analytical forms for thermal resistance and heat rate. Consequences For a cylinder: Q = 2πkL (T1 − T2) / ln(r2/r1). For a sphere: Q = 4πk (T1 − T2) / (1/r1 − 1/r2). Final Answer False — the conduction relations are different for thick spheres and thick cylinders.

Question 8

Psychrometrics relation: dew-point vs wet-bulb as relative humidity decreases  As the relative humidity of moist air decreases, how does the dew-point temperature compare to the wet-bulb temperature? Choose the correct relation.

Accepted Answer

Given concept For unsaturated air: Tdew ≤ Twb ≤ Tdb. As relative humidity (RH) drops, the mixture is further from saturation, so Tdew falls away from Twb. Reasoning Lower RH → lower vapor partial pressure → lower Tdew. The wet-bulb temperature decreases too, but not as much; thus Tdew stays below Twb. Common pitfall Confusing Tdew with Twb; they are equal only at saturation (RH = 100%). Final Answer Less than the wet-bulb temperature.

Question 9

Heat transfer truths and one false statement: identify the incorrect claim  Four statements about heat transfer and temperature differences are given. Select the statement that is wrong.

Accepted Answer

Concept check (A) True in general: convection dominates bulk heat transfer in fluids (though conduction at the boundary initiates it).(B) True: heat flow depends on thermal conductivity k (a material property) and geometry.(D) True: LMTD ≠ arithmetic mean; LMTD accounts for the varying temperature difference along the exchanger. Why (C) is wrong For most solid metals, lattice/electron scattering increases with temperature, so thermal conductivity decreases as temperature rises (over wide ranges of practical interest). Final Answer “For solid metals, thermal conductivity increases with temperature” is the incorrect statement.

Question 10

Adiabatic saturation of unsaturated air: identify the quantity that remains constant  During an adiabatic saturation process applied to unsaturated air (idealized psychrometric analysis), which standard air property stays essentially constant? Choos…

Accepted Answer

Approach In the ideal adiabatic saturation process (no heat/work interactions with surroundings; negligible KE/PE changes), the air-vapor mixture follows an approximately constant wet-bulb temperature (and constant enthalpy) path on the psychrometric chart. Eliminations Relative humidity varies as the state approaches saturation.Dew-point and dry-bulb temperatures change during adiabatic humidification. Final Answer Wet-bulb temperature remains essentially constant.

Question 11

The centrifugal compressors are generally used for refrigerants that require

Accepted Answer

large displacements and low condensing pressures

Question 12

Which of the following statement is correct for ammonia as a refrigerant?

Accepted Answer

all of these

Question 13

During humidification process, __________ increases.

Accepted Answer

relative humidity

Question 14

Conduction is a process of heat transfer

Accepted Answer

from one particle of the body to another without the actual motion of the particles

Question 15

Sulphur dioxide is extremely toxic and non-flammable.

Accepted Answer

True

Question 16

A heat pump working on a reversed Carnot cycle has a C.O.P. of 5. It works as a refrigerator taking 1 kW of work input. The refrigerating effect will be

Accepted Answer

4 kW

Question 17

The emissivity for a black body is

Accepted Answer

1

Question 18

Thermal conductivity of air increases with rise in temperature.

Accepted Answer

Correct

Question 19

In a heat exchanger with one fluid evaporating or condensing, the surface area required is least in

Accepted Answer

all of these

Heat Transfer, Refrigeration and Air Conditioning Questions