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Cold Air-Standard Assumptions

Hot Reservoir

Cold Reservoir

The Air-Standard Assumptions

The working fluid is air and it behaves as an ideal gas
The cycle is modeled as a closed cycle with the air cooled in heat exchanger #2 and recirculated to the compressor.
The combustion reactor is replaced by heat exchanger #1
All processes are internally reversible

The heat capacities of air are constant and have the values determined at 25°C.

Lesson 10E: page 2 of 7

Here is a quick review of the air-standard assumptions that we used in the analysis of the air standard Brayton Cycle.
We will use all of the same assumptions in our analysis of the air-standard refrigeration cycle.
All except #3. There isn’t a combustion reactor in a refrigerator ! Not in mine, anyway !
Instead of a combustion reactor, we have an ordinary hot reservoir.
In most cases, the hot reservoir is the surroundings. You just can’t beat the cost … free !
The cold air-standard assumption just makes all the calculations more simple.
We assume that the heat capacities of air are constant and always have the value at 25oC.
This certainly makes integrating the Gibbs equations easier !
The cycle that we are going to analyze based on these assumptions is made of 4 steps.
Step 1-2: Isobaric heating by absorbing heat from the cold reservoir
Step 2-3: Isentropic compression from the low pressure of HEX #2 to the high pressure of HEX #1.
Step 3-4: Isobaric cooling by rejecting heat to the hot reservoir.
Step 4-1: Isentropic expansion in the turbine from the high pressure of HEX #1 down to the low pressure of HEX #2.
The process moves counterclockwise around the cycle like all refrigeration cycles.
The cycle shown here is an ideal cycle.
On the next page we discuss the process path for a real gas refrigeration cycle.