Reference no: EM132584221

ME321 Refrigeration and Air Conditioning Assignment -

Part A -

Q1. A Bell Coleman refrigeration cycle using air as the working fluid has a pressure ratio of 4. Air enters the compressor at -7^oC. The high pressure air is cooled to 27^oC by rejecting heat to the surroundings. It is further cooled to -15^oC by regenerative cooling before it enters the turbine. Assuming both the turbine and compressor to be isentropic, determine (a) the lowest temperature that can be obtained by this cycle, (b) the COP of the cycle and (c) mass flow rate of air for a refrigeration rate of 12 kW.

Hint: The regenerative heat exchanger is used to exchange heat between gas streams entering the turbine and compressor.

Q2. An airplane using 20 TR bootstrap air refrigeration system has ambient conditions of 0.9 bar and 15^oC. Ram air pressure after isentropic compression is 1.1 bar. Main compressor exit pressure is 3.5 bar, and exit pressure of secondary compressor is 4.5 bar. The cabin is required to be maintained at 1 bar and 25^oC. Isentropic efficiency of each compressor is 85% and that of cooling turbine is 90%. The effectiveness of both heat exchangers is 60%. Find (a) mass flow rate of air passing through the cabin, (b) power required and (c) COP of the system. Assume c_p = 1.01 kJ/kgK.

Q3. In a steam-jet refrigeration system, dry and saturated steam at 7 bar enters the ejector. The temperature of the evaporator (flash chamber) is 4^oC and make-up water is 17^oC. The condenser pressure is 0.06 bar. The nozzle, entrainment and diffuser efficiencies are 0.9, 0.6 and 0.7, respectively. Assume the quality of the steam at the entry of diffuser to be 0.92 and neglect the normal shock. Determine the mass flow rate of motive steam per kg of flash vapour, refrigeration effect and the COP.

Q4. An aqua-ammonia absorption refrigeration system has an evaporator temperature of -12^oC and a condenser temperature of 50^oC. Heat is supplied to the generator by condensing steam at 4 bar, 90% quality. Estimate maximum COP of the system.

If the actual COP is 30% of the maximum COP and the refrigeration load is 30 TR, calculate the steam flow rate.

Part B -

Q1. An air-cooling system for a jet plane operates on the simple cycle. The cabin is to be maintained at 25^oC and 1.01325 bar. The ambient air pressure and temperature are 0.35 bar and -15^oC, respectively. The pressure after jet compression is 2 bar and the plane is flying at 0.2 Mach. The airplane cooling load is 30 TR. Using EES or any programming language, plot the behavior of

(a) Dry Air Rated Temperature

(b) Mass flow rate of air circulated

(c) Coefficient of performance

with Mach number varying between 0.2 and 2. Using the results, determine the maximum attainable speed for the simple system.

Q2. Repeat this problem for

(i) Bootstrap system with secondary compressor exit pressure as 3 bar.

(ii) Reduced ambient system.

Consider the efficiency of all expanders and compressors as 90% and 85%, respectively. The ram efficiency can be estimated as 0.85 and effectiveness of all heat exchangers as 0.8. Draw a schematic diagram for all three air refrigeration systems clearly marking the inlet and outlets of each component along with T - s plots. Discuss the factors prompting higher accessible mach numbers for the reduced ambient system.

Hint: COP can be calculated as the ratio of cooling load to compressor work only (ignore expander work) for the comparison between 3 air-refrigeration systems.