# Cold Air-Standard Assumptions

Hot Reservoir
QH
Cold Reservoir
QC

## The Air-Standard Assumptions

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

## Cold Air-Standard Assumption

The heat capacities of air are constant and have the values determined at 25°C.
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### Ch 10, Lesson E, Page 2 - Cold Air-Standard Assumptions

• 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.