Heat Transfer, Refrigeration and Air Conditioning Questions

• Reading the psychrometric chart Which set of lines on a standard psychrometric chart are vertical and uniformly spaced, and what do they represent?
• Equivalent insulation thickness A furnace wall of red brick (thermal conductivity k₁ = 0.7 W/m·K) has thickness 0.50 m. For the same heat loss and the same temperature drop across the wall, what thickness of diatomite earth (thermal conductivity k₂ = 0.14 W/m·K) would provide an equivalent thermal resistance?
• Relative performance comparison Compared with a typical domestic air-conditioner, the coefficient of performance (C.O.P.) of a domestic refrigerator is generally:
• Psychrometric processes — dehumidification path On a standard psychrometric chart, which line best represents an ideal dehumidification-at-constant-dry-bulb process (e.g., chemical dehumidification)?
• Modes of heat transfer — microscopic mechanism Heat transfer that occurs primarily through molecular collisions within a material (without bulk motion) is called:
• Heat transfer from a hot cube to a constant-temperature bath: If the cube loses heat from the top, bottom, and the four side faces with surface heat transfer coefficients h1, h2, and h3 respectively, what is the area-averaged overall heat transfer coefficient for the entire cube?
• Psychrometrics check: During sensible cooling of air (no moisture removed), the relative humidity of the air stream will __________.
• Radiation fundamentals (Wien’s displacement): The wavelength corresponding to the peak (maximum) emissive power of a blackbody varies inversely with absolute temperature (λ_max * T = constant). The blank in “wavelength corresponding to __________” should be filled with:
• Refrigeration cycles comparison: The coefficient of performance (COP) of a vapour-compression refrigeration system is generally __________ than that of an air (gas) refrigeration system operating between similar temperature limits.
• Sensible heating coil performance: For air heated sensibly across a coil, the coil efficiency (or effectiveness) in terms of the by-pass factor (B.P.F.) is:
• Psychrometric limit: At 100% relative humidity, the wet-bulb temperature is __________ the dry-bulb temperature.
• Air-conditioning process metric: The sensible heat factor (SHF) for an air-conditioning process is defined as (use S.H. = sensible heat and L.H. = latent heat over the process):
• Reading the psychrometric chart: The horizontal, non-uniformly spaced lines on a standard psychrometric chart correspond to which property?
• Cooling and dehumidification requirement: To cool and dehumidify a moist air stream across a coil, the coil surface temperature must be set __________ the incoming air’s dew-point temperature.
• Refrigerant safety comparison: Which of the following refrigerants is both highly toxic and flammable under typical service conditions?
• HVAC duct sizing — choose the most widely used design approach In practical air-distribution design for buildings, several sizing methods exist (velocity reduction, equal friction, static regain, and dual methods). Which method is most commonly adopted for initial duct sizing because of its simplicity and predictable pressure-drop control?
• Refrigerant selection — identify the undesirable property For a refrigerant used in vapor-compression systems, which of the following is an undesirable thermophysical or safety property for practical HVAC/R applications?
• Radiation fundamental — name the law: Q = σ A T^4 Identify the classical radiation relation that states total emissive power of a black surface varies with the fourth power of absolute temperature.
• Heat rejection factor (HRF) for R-12/R-22 at 40°C condenser and 5°C evaporator In typical comfort-cooling applications with R-12 or R-22 and operating conditions Tc ≈ 40°C and Te ≈ 5°C, the heat rejection factor (ratio of condenser heat rejected to net refrigeration effect) is approximately:
• Psychrometrics concept check “The heating of air without any change in its specific humidity is called sensible heating.” State whether the statement is correct.