(SEM VII) THEORY EXAMINATION 2024-25 HVAC SYSTEMS

SECTION A

(2 × 10 = 20 marks | Short Answers)

a) Effect of superheating on COP of a vapour compression cycle

Superheating increases the refrigerating effect per kg of refrigerant, which can improve COP slightly. However, excessive superheating increases compressor work and discharge temperature, which may reduce COP. Hence, limited superheating near the compressor inlet is beneficial, but too much is harmful.

b) Hydrofluoro-Olefins (HFOs) and why they are future refrigerants

HFOs are fourth-generation refrigerants with very low Global Warming Potential (GWP) and zero Ozone Depletion Potential (ODP). They break down quickly in the atmosphere and meet modern environmental regulations, making them suitable replacements for HFCs.

c) Significance of Apparatus Dew Point (ADP)

ADP is the effective surface temperature of a cooling coil. It represents the lowest temperature air can reach by contact with the coil. ADP helps in determining cooling capacity, dehumidification, and coil performance.

d) Bypass Factor of a cooling coil

Bypass factor is the fraction of air that does not come into contact with the cooling coil.

Bypass Factor=Tout−TADPTin−TADP\text{Bypass Factor} = \frac{T_{out} - T_{ADP}}{T_{in} - T_{ADP}}Bypass Factor=Tin​−TADP​Tout​−TADP​​

Lower bypass factor means better cooling and dehumidification.

e) Unitary vs Central Air Conditioning

Unitary system: Individual rooms, small capacity, low initial cost

Central system: Large buildings, centralized control, higher efficiency, higher initial cost

f) Purpose of mechanical ventilation

Mechanical ventilation supplies fresh air, removes contaminants, odors, and excess heat, and maintains indoor air quality where natural ventilation is insufficient.

g) Shading device and its role

Shading devices (louvers, fins, overhangs) block direct solar radiation, reducing heat gain through windows and lowering cooling load in buildings.

h) Stack effect

Stack effect is air movement due to temperature difference between indoor and outdoor air, causing warm air to rise and escape, increasing infiltration load in buildings.

i) Equal friction method

A duct design method where pressure loss per unit length is kept constant throughout the duct system, ensuring uniform air distribution and economical duct sizing.

j) Centrifugal vs Axial fans

Centrifugal fan: High pressure, low flow, noisy, used in HVAC ducts

Axial fan: High flow, low pressure, compact, used for ventilation

SECTION B

(Attempt any 3 | 10 marks each)

a) Transcritical vapour compression refrigeration cycle

In a transcritical cycle (used with CO₂), the refrigerant is compressed above its critical pressure. Heat rejection occurs in a gas cooler instead of a condenser. The cycle is represented on a P-h diagram showing compression, gas cooling, expansion, and evaporation. It offers high efficiency in cold climates.

b) Psychrometric processes in summer air conditioning

Summer air conditioning involves:

Cooling and dehumidification (air passes over cooling coil)

Reheating (if required for comfort)

Mixing of return and fresh air
Psychrometric chart is used to analyze temperature, humidity, and enthalpy changes.

c) Decentralized heat pumps in industries

Advantages:                                                                    Flexible installation

Energy efficient                                                                 Reduced transmission losses

Limitations:                                                                     High initial cost

Maintenance complexity                                                   Limited capacity for very large industries

d) Heat transfer through walls using decrement factor

Decrement factor accounts for time lag and attenuation of heat flow through walls due to thermal mass.

Q=U×A×(To−Ti)×Decrement FactorQ = U \times A \times (T_{o} - T_{i}) \times \text{Decrement Factor}Q=U×A×(To​−Ti​)×Decrement Factor

Used in cooling load calculations for buildings.

e) Velocity reduction method for duct design

Air velocity is gradually reduced as air moves away from the fan. This method reduces noise and friction losses and is commonly used in long duct systems.

SECTION C

Q3 (Attempt any one)

a) Classification of refrigerants based on safety

ASHRAE classifies refrigerants as:

A1: Non-toxic, non-flammable (R134a)

A2/A3: Low/high flammability

B: Toxic refrigerants (Ammonia – B2)

b) Maximum COP in superheated region

For isobutane at given temperatures, thermodynamic analysis shows that maximum COP occurs when suction state lies in the superheated region, due to increased refrigeration effect.

Q4 (Attempt any one)

a) Air washer with hygroscopic solution

An air washer sprays hygroscopic solution (like lithium chloride) to control humidity. It absorbs moisture from air in summer and adds moisture in winter, providing humidification and dehumidification.

b) Steam injection humidification

Using psychrometric relations, final state of air is determined by locating initial DBT and humidity ratio, then moving vertically upward to new humidity ratio on the psychrometric chart.

Q5 (Attempt any one)

a) Double-bundle condenser

It consists of two tube bundles: one for heating water and the other for heat rejection. Used in heat pumps to improve efficiency and provide simultaneous heating.

b) Ammonia compression system numerical

Using ammonia tables:

Compressor power is calculated from enthalpy difference

COP = Refrigerating effect / Compressor work

Q6 (Attempt any one)

a) Heat gain through glass

Heat gain = Solar radiation + conduction + convection
Calculated using SHGF, shading coefficient, and glass area.

b) Energy conservation in AC buildings

Includes insulation, efficient HVAC equipment, LED lighting, proper ventilation, and smart controls to reduce energy consumption.

Q7 (Attempt any one)

a) Flow through supply air outlets

Air spreads due to jet action, entrainment, and pressure difference, ensuring uniform room air distribution.

b) Vibration and noise in AC piping

Causes: High velocity, poor supports, compressor vibration
Solutions: Flexible joints, vibration isolators, proper pipe supports