MECH5255: AIR CONDITIONING AND REFRIGERATION Assignment 2
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MECH5255: AIR CONDITIONING AND REFRIGERATION
Assignment 2: Refrigeration cycles and Psychrometrics
Q1:
An ice cream making dairy plant requires a refrigeration system that provides 4150 kW of cooling capacity at -2.00°C to cool milk and 3150 kW of cooling capacity at -28.0°C to fast chill the ice cream . The system condenser pressure is 1 .20 MPa and is an air-cooled condenser. A multi- pressure system is proposed with two compressors and two evaporators, utilising flash gas removal and inter-cooling as per Fig. 1. Assuming all compressors have an isentropic efficiency of 90.5%, there is no heat transfer in connecting components/pipework, the flashtank intercooler issues saturated vapour and saturated liquid in the respective streams, there is no pressure drop in the pipework, 4.00°C of subcooling in the condenser, and 2.00°C of superheating in both evaporators and R134a* is the refrigerant, determine:
i) The mass flow rates of the refrigerant through the evaporators and condenser . (5 marks)
ii) The power requirements for the compressors. (5 marks)
iii) It is proposed that the capital cost of the system could be reduced by $28,000 if two separate single-stage systems with air-cooled condensers rather than a single multi- pressure system were utilised. Assuming the lifetime of the system to be 9.0 years, the system runs 18 hrs a day with 90.0% uptime and electricity costs are $0.40/kW.hr, which is the better approach: the multi- pressure system or two independent systems? Why? (10 marks)
iv) The cooling requirement for the high-temperature (-2.00°C) evaporator has the potential to vary through the day. Construct a model of the multi- pressure system in Matlab using the NISTref prop library and create a figure of total electrical power input vs. the high-temperature (-2.00°C) evaporator cooling heat absorption rates from 1000 to 5000 kW. Note at a heat absorption rate of 3150 kW the total power input should be close to the value obtained in part ii). The idea is to use part iv) to check your hand calculation results so do not use part iv) as a replacement for the hand calculation answer in part ii) of the question. You must include a well-documented copy of the code you develop and utilise. (30 marks)
Q2:
Conditioned air is to be supplied to a space that requires a dewpoint of 16.00°C and 50.0% relative humidity to be maintained. The space is to be conditioned by cooling a mixture of 55.0% return air (by volume) and outside atmospheric air at 1 atm, 38.0°C dry bulb and 38% relative humidity in a constant pressure process. The space load has a sensible heat factor of 0.720 and the coil-off temperature is 12.00°C . The flow rate to the space is 35,000 L/min. For parts i), ii), and iii) of this question utilise the psychrometric chart for your solution.
i) Sketch the system hardware and show the process on a psychrometric chart. Show the process on the psychrometric chart for a hypothetical real cooling process over the coil and an ideal wetted cooling process over the cooling coil. (5 marks)
ii) Determine the dew- point temperature and the volume flow rate of the mixture entering the cooling coils, the apparatus dew point (if it exists), and the coil off condition wet bulb temperature. (9 marks)
iii) Determine the total coil cooling load. (3 marks)
iv) Determine the room sensible and latent heat loads. (3 marks)
v) During the day the outside dry bulb temperature varies between 34.0°C and 42.0°C, assuming the relative humidity remains constant, create a plot using Matlab, Excel or other software of the total coil, sensible and latent cooling load vs. outside dry bulb temperature. Note that the total coil cooling load value at an outside dry bulb temperature of 38.0°C should be very close to the value you obtain in part iii). Assume all other input parameters for the problem remain constant. You will need to use exact methods utilising the saturated water properties, ideally use an equation such as the Arden- Buck equation for the saturated water vapor pressure, or alternatively the table properties from the textbook*. It is emphasised you should not be taking values from the psychrometric chart to answer this part of the question. In your submission, you must include a copy of all code used, the code must be well presented and well documented. If Excel is used you must show the formulas used in each cell. Using Matlab is highly recommended but not mandatory. (30 marks)
Fig. 1.
*Utilise the tables provided on Canvas for R134a which are taken from “Thermodynamics and engineering approach”, seventh edition in SI units, Cengel and Boles . For all calculations assume linear interpolation of these tables is acceptable.
**The tables provided on Canvas for water which are taken from “Thermodynamics and engineering approach”, seventh edition in SI units, Cengel and Boles . If these tables are used, assume linear interpolation of these tables is acceptable.
2022-10-06