Refrigeration and Air Conditioning - Dec 2014
Mechanical Engineering (Semester 7)
TOTAL MARKS: 100
TOTAL TIME: 3 HOURS (1) Question 1 is compulsory.
(2) Attempt any four from the remaining questions.
(3) Assume data wherever required.
(4) Figures to the right indicate full marks.
Attempt any four:
1 (a) Define Refrigeration. What are the applications of refrigeration in:
i) Food Processing
iii) Industrial Sector(5 marks) 1 (b) A machine working on a Carnot cycle operates between 310K and 250K. Determine COP when it is operated as
i) A refrigerating machine
ii) A heat pump(5 marks) 1 (c) Explain the concept of Effective Temperature. Which are the factors governing effective temperature?(5 marks) 1 (d) Explain working of simple VCR cycle with schematic P-h and T-s diagram.(5 marks) 1 (e) Explain equal pressure drop method used in duct designing.(5 marks) 2 (a) What is standard commercial ton of refrigeration.(4 marks) 2 (b) A regenerative air cooling system is used for an airplane to take 20 TR of refrigeration load. The ambient air at pressure 0.8 bar and temp 10 C is rammed isentropically till the pressure rises to 1.2 bar. The air bled off the main compressor at 4.5 bar is cooled by the ram air in the heat exchanger whose effectiveness is 60%. The air from the heat exchanger is further cooled to 60 C in the regenerative heat exchanger with a portion of air bled after expansion in the cooling turbine. The cabin is to be maintained at a temp of 25 C and a pressure of 1 bar. If the isentropic efficiencies of the compressor and turbine are 90 % and 80% respectively; find
i) Mass of air bled from cooling turbine to be used for regenerative cooling;
ii) Power required for maintaining the cabin at the required condition; and
iii) COP of the system.
Assume the temperature of air leaving to atmosphere from the regenerative heat exchanger as 100C.(12 marks) 2 (c) Draw and explain schematic diagram and T-S diagram for the Boot strap air cooling system.(4 marks) 3 (a) Explain with neat schematic; liquid to vapour heat exchanger used in vapour compression refrigerating system.(6 marks) 3 (b) A vapour compression system using R12 works between -15 C and 35 C on the evaporator and condenser sides respectively. Using P-h chart determine:
ii) Mass flow rate of refrigerant per TR,
iii) Piston displacement per TR using volumentric efficiency of 80%.
iv) Heat rejected in the compressor per TR, and
v) Ideal COP.(10 marks) 3 (c) Explain cascade refrigerating system?(4 marks) 4 (a) Classify different types of CompressoRs. Explain each type in brief.(6 marks) 4 (b) What are the types of expansion valves? Explain the working of thermostatic expansion valve with diagram.(6 marks) 4 (c) Draw a neat diagram of LiBr-Water absorption system and explain its working. List the major field applications of this system.(8 marks) 5 (a) The room sensible and latent heat loads, for an air conditioned space are 25 kW and 7.5 kW respectively. The room condition is 26 C dry bulb temperature and 50% relative humidity. The outdoor condition is 40 C dry bulb temperature and 50% relative humidity. The ventilation requirement is such that on mass flow rate basis 25% of fresh air is introduced and 75% of supply ait is recirculated. The by-pass factor of the cooling coil is 0.15. Determine i) Supply air flow rate ii) Outside air sensible heat iii) Outside air latent heat and iv) Grand total heat.(10 marks) 5 (b) The readings from a sling psychrometer are as follows:
Dry Bulb Temperature 34C
Wet Bulb Temperature 24 C
Barometer reading: 740 mm Hg
Without using psychrometic Chart, calculate:
i) Dew Point Temperature
ii) Relative Humidity
iii) Specific Humidity
iv) Degree of Saturation
v) Enthalpy of mixture per Kg of dry air.(10 marks) 6 (a) Define Ton of Refrigeration. If the COP of a 1.5 TR Window Air Conditioner is 3.3 calculate how much power will the compressor consume?(5 marks) 6 (b) What is fog? Show on the psychrometric chart when two air streams yield fogged state of air.(8 marks) 6 (c) Define RSHF and ERSHF. Explain how to draw RSHF, GSHF and ERSHF lines on the psychrometric chart.(6 marks)
Write short notes any four:
7 (a) Dry Ice and its applications.(5 marks) 7 (b) Deep sea water air conditioning.(5 marks) 7 (c) Noise and its control in A/C system.(5 marks) 7 (d) Liquefaction of gases.(5 marks) 7 (e) Defrosting(5 marks)