ME-801 Refrigeration and Air Conditioning - May 2022 Question Paper

Under each question, How to answer lists the points an answer should cover, outlined by CampusPrep from the syllabus. Check the details against your textbook.

1. 7 Marks
   aExplain the throttling processes and Joule Thomson effect for cooling.
   How to answer

   • Define throttling as isenthalpic expansion through restriction
   • Explain Joule-Thomson coefficient mu = (dT/dp)_h and inversion temperature
   • Explain cooling condition mu positive below inversion temperature
   • State application to refrigeration / liquefaction
2. 7 Marks
   bIn an ideal vapour compression refrigeration cycle, the enthalpy of the refrigerant at exit from the condenser, compressor and evaporator is $80\text{ kJ/kg}$, $200\text{ kJ/kg}$ and $180\text{ kJ/kg}$ respectively. What is the coefficient of performance of the cycle? Draw the P-V, T-S and P-h Diagram for the above cycle.
   How to answer

   • Compute refrigerating effect = h_evaporator_out - h_condenser_out
   • Compute work = h_compressor_out - h_evaporator_out
   • Compute COP = RE / W
   • Sketch P-V, T-S and P-h diagrams for vapour compression cycle
3. 7 Marks
   aExplain dry ice? Write a short note on production of dry ice.
   How to answer

   • Define dry ice as solid carbon dioxide
   • Give properties - sublimation at -78.5C
   • Describe production method - expansion / pressing of CO2 snow
   • Mention uses
4. 7 Marks
   bThe ambient air temperatures during summer and winter in a particular locality are $45^{\circ}C$ and $15^{\circ}C$ respectively. Find the values of Carnot COP for an air conditioner for cooling and heating, corresponding to refrigeration temperatures of $5^{\circ}C$ for summer and heating temperature of $55^{\circ}C$ for winter. Assume suitable temperature differences in the exchanger that exchanges heat with the surroundings. If water from the cooling tower at $30^{\circ}C$ is used as a cooling medium with $3^{\circ}C$ temperature differential for air-conditioning in summer, what will be the Carnot COP for cooling? Also, find the theoretical power consumption per ton of refrigeration in each case. Assume no increase in the temperature of the surrounding air or water.
   How to answer

   • Assume temperature differences in heat exchangers to get cycle temperatures
   • Calculate Carnot COP cooling = TL/(TH-TL) and heating = TH/(TH-TL)
   • Repeat with cooling tower water at 30C + 3C differential
   • Calculate theoretical power per ton = 3.517/COP kW/TR
5. 7 Marks
   aA refrigerator storage is supplied with $3600\text{ kg}$ of fish at a temperature of $27^{\circ}C$. The fish has to be cooled to $-23^{\circ}C$ for preserving it for a long period without deterioration. The cooling takes place in $10\text{ hours}$. The specific heat of fish is $2.0\text{ kJ/kgK}$ above freezing point of fish and $0.5\text{ kJ/kgK}$ below freezing point of fish, which is $-3^{\circ}C$. The latent heat of freezing is $230\text{ kJ/kg}$. What is the power to drive the plant if the actual COP is half that of the ideal COP?
   How to answer

   • Calculate sensible + latent + subcooling loads for 3600 kg fish
   • Find total refrigeration load / time for ideal COP
   • Use Carnot COP between relevant temperatures then actual COP = half ideal
   • Compute power = refrigeration load / actual COP
6. 7 Marks
   bWhat is the Required Properties of Ideal Refrigerant?
   How to answer

   • List thermodynamic requirements - low boiling point, high latent heat, high COP
   • List physical/chemical - stability, non-corrosive, non-toxic, low viscosity
   • List safety and environmental - ODP, GWP, flammability
   • Summarize ideal refrigerant characteristics
7. 7 Marks
   aWrite down the short note on Li-Br Refrigerant Cycle with Neat Sketch.
   How to answer

   • Describe Li-Br vapour absorption components - generator, absorber, condenser, evaporator, heat exchanger
   • Explain working with Li-Br solution as absorbent and water as refrigerant
   • Provide neat labelled sketch
   • Mention applications and COP
8. 7 Marks
   bExplain in brief which refrigerant/s would you choose for each of the following applications and why?

   1. i)A cold storage of 100 TR capacities using reciprocating compressor.
   2. ii)An 800 TR air conditioning plant using centrifugal compressor/s.
   3. iii)A small capacity frozen food cabinet to maintain $-30^{\circ}C$ temperature
   How to answer

   • For 100 TR reciprocating cold storage - suggest ammonia / R-22 and reasons
   • For 800 TR centrifugal AC plant - suggest R-11 / R-12 / low pressure refrigerant and reasons
   • For small -30C frozen food cabinet - suggest R-12 / R-502 / hydrocarbon and reasons
   • Compare on basis of pressure, toxicity, COP, compressor suitability
9. 7 Marks
   a$39.6\text{ cmm}$ of a mixture of recirculated room air and outdoor air enter a cooling coil at $31^{\circ}C$ DB and $18.5^{\circ}C$ WB temperatures. The effective surface temperature of the coil is $4.4^{\circ}C$. The surface area of the coil is such as would give $12.5\text{ kW}$ of refrigeration with the given entering air state. Determine the dry and wet bulb temperatures of the air leaving the coil and the coil bypass factor.
   How to answer

   • Determine entering air enthalpy from 31C DB 18.5C WB using chart
   • Use coil capacity 12.5 kW and air flow 39.6 cmm to find leaving enthalpy
   • Locate leaving state on saturation / bypass line with ADP 4.4C
   • Find leaving DB, WB and bypass factor = (t1-t2)/(t1-ts)
10. 7 Marks
    bWrite down the short note on comfort air conditioning? What are the factors affecting comfort air conditioning?
    How to answer

    • Define comfort air conditioning and comfort zone
    • Explain role of temperature, humidity, air motion, purity
    • List factors - DBT, RH, air velocity, clothing, activity
    • Mention ASHRAE comfort chart
11. 14 Marks
    Moist air enters a chamber at $5^{\circ}C$ DBT and $2.5^{\circ}C$ thermodynamic WBT at a rate of $90\text{ cmm}$. The barometric pressure is $1.01325\text{ bar}$. While passing through the chamber, the air absorbs sensible heat at the rate of $40.7\text{ kW}$ and picks up $40\text{ kg/h}$ of saturated steam at $110^{\circ}C$. Determine the dry and wet bulb temperatures of the leaving air.
    How to answer

    • Find entering air specific volume, mass flow from 90 cmm at 5C DB 2.5C WBT
    • Apply steady flow energy and mass balance with 40.7 kW sensible heat and 40 kg/h steam at 110C
    • Solve for leaving humidity ratio and enthalpy
    • Read leaving DBT and WBT from psychrometric relations
12. 14 Marks
    The air-handling unit of an air-conditioning plant supplies a total of $4500\text{ cmm}$ of dry air which comprises by weight $20$ percent fresh air at $40^{\circ}C$ DBT and $27^{\circ}C$ WBT, and $80$ percent recirculated air at $25^{\circ}C$ DBT and $50$ percent RH. The air leaves the cooling coil at $13^{\circ}C$ saturated state. Calculate the total cooling load and room heat gain.

    How to answer

    • Calculate mixture condition from 20% outside and 80% inside by weight using given table
    • Find mass flow of dry air from 4500 cmm
    • Compute total cooling load = m*(h_mix - h_ADP)
    • Compute room heat gain = m*(h_inside - h_ADP)
13. 14 Marks
    Write a short note on any three:

    1. a)Comfort air conditioning
    2. b)Leak detection methods
    3. c)Electrolux
    4. d)Cascade system
    How to answer

    • Select any three topics from list
    • Give definition, working principle and key components for each
    • Add neat sketch / types where applicable
    • Mention applications