(SEM VII) THEORY EXAMINATION 2022-23 HVAC SYSTEMS

SECTION A – Short Answers (2 Marks Each)

(a) Chemical dehumidification

Chemical dehumidification removes moisture from air using hygroscopic substances like silica gel, calcium chloride, or lithium chloride which absorb water vapour chemically.

(b) Removal of moisture from a refrigeration system

Moisture is removed using dehydrators, filter dryers, vacuum evacuation, and proper charging of dry refrigerant.

(c) Significance of alignment circle on psychrometric chart

The alignment circle helps in determining properties such as sensible heat factor, mixing of air streams, and cooling/heating processes accurately.

(d) Effective Sensible Heat Factor (ESHF)

ESHF is the ratio of effective sensible heat to the total effective heat load, considering both sensible and latent heat effects.

(e) Auto refrigeration

Auto refrigeration is cooling produced due to pressure drop in a fluid without external heat transfer, commonly seen in throttling processes.

(f) Effective temperature and its utility

Effective temperature is an index combining air temperature, humidity, and air velocity to represent human comfort level in air conditioning.

(g) Human comfort according to ASHRAE

ASHRAE defines human comfort as a condition of mind that expresses satisfaction with the surrounding thermal environment.

(h) Importance of sol-air temperature

Sol-air temperature accounts for solar radiation and ambient air temperature and is used to estimate heat gain through building walls and roofs.

(i) Preferred duct type in air conditioning

Rectangular sheet-metal ducts are commonly preferred due to ease of fabrication and installation.

(j) Specific speed of centrifugal fan

Specific speed is defined as the speed at which a geometrically similar fan would operate to deliver unit flow against unit head.

SECTION B – Long Answers (10 Marks Each)

(a) Desirable properties of an ideal refrigerant

An ideal refrigerant should have high latent heat, low boiling point, non-toxicity, non-flammability, chemical stability, low cost, environmental safety, and good compatibility with lubricants.

(b) Mechanical ventilation vs natural ventilation

Mechanical ventilation offers better air control, filtration, and reliability but requires power and higher initial cost. Natural ventilation is economical but unreliable and weather-dependent.

(c) Factors affecting human comfort

Human comfort depends on air temperature, relative humidity, air velocity, radiant temperature, clothing, activity level, and air purity.

(d) Heat loads of a large restaurant

Heat loads include external loads (walls, roofs, windows), internal loads (occupants, lighting, equipment), ventilation load, infiltration load, and latent heat due to cooking.

(e) Methods of air-conditioning duct design

Common methods include equal friction method, static regain method, velocity reduction method, and constant velocity method.

SECTION C – Long Answers (10 Marks Each)

3(a) COP of refrigerating system with three evaporators

For three evaporators operating at the same temperature with a single compressor, the total refrigeration effect equals the sum of individual evaporator loads.
COP = Total refrigeration effect / Compressor work input.

3(b) Azeotropic and non-azeotropic mixtures

Azeotropic mixtures behave like pure substances with constant boiling point (e.g., R-502). Non-azeotropic mixtures have temperature glide during phase change and offer better efficiency.

4(a) Industrial applications of air conditioning

Air conditioning is used in textile industries, pharmaceutical plants, printing presses, electronic manufacturing, food processing, and chemical industries to control temperature and humidity.

4(b) Underground heat pump

An underground heat pump uses earth as a heat source or sink. Heat is extracted or rejected through buried pipes, offering high efficiency and energy savings.

5(a) Comparison of air-conditioning systems

All-air systems use air for cooling and ventilation, all-water systems use chilled water, and air-water systems combine both for flexibility and efficiency.

5(b) Unitary vs central air conditioner

Unitary systems are compact and serve single spaces, while central air conditioners serve large buildings using centralized equipment and duct networks.

6(a) Heating of moist air (Numerical – Method)

Heat added is calculated using enthalpy difference from psychrometric chart.
Final RH decreases, while WBT increases slightly after sensible heating.

6(b) Steam heating coil (Numerical – Method)

Sensible heat transfer is calculated using change in enthalpy of air.
Steam flow rate is obtained using energy balance on steam condensation.

7(a) Adiabatic mixing of air streams (Numerical – Method)

Dry-bulb temperature, wet-bulb temperature, RH, and specific humidity are determined using mass and energy balance with psychrometric relations.

7(b) Cooling load calculations for bank building

Grand total heat = sensible + latent heat.
ESHF, apparatus dew point, and air volume flow rate are calculated using psychrometric relations and bypass factor.