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All refrigerants have a pressure-enthalpy diagram. In short, the diagram shows property conditions of the refrigerant at various stages of state.
The properties inside the “thumbprint” shape is “saturation” properties (mixtures of liquid and vapor). The area to the left of the thumbprint are properties of liquid refrigerant, and to the right, properties of vapor refrigerant.
The left vertical axis is the index for pressure (psia, pounds per square foot atmospheric). The bottom axis is the index for enthalpy, or energy, the capacity of the refrigerant at that stage. Other lines, temperature (shown), volume (not shown), entropy (not shown), and quality (not shown), allow for the state of the refrigerant in a cycle to be plotted to find unknown values that can be used to calculate energy, cooling capacity, efficiency, horse power, and many more valuable bits.
Using the pressure and temperature from a system the cycle below was plotted. I want to know how much horse power I should expect the system to be using.
The two red lines in the diagram are drawn to be straight drops for the intersection of the cycle point, compressor inlet, and compressor discharge. The lines cross the enthalpy line at 177.5 and 188.5 (approximately).
By subtracting 177.5 from 188.5, I arrive at a specific enthalpy of compression of 11 Btu per pound of refrigerant compressed per minute of operation.
The formula to find our horse power is:
42.43 is a mechanical constant, the mass flow of refrigerant we will find next.
Let’s say we are working on a 2-ton air-conditioner, and since 200 Btu per minute equals 1 ton of cooling, our total cooling per minute needed is 200 X 2 = 400 Btu per minute.
Since each refrigerant, and set of conditions, have a specific cooling capacity called the “net refrigeration effect” we must find the specific cooling capacity of the system plotted above.
To do this, we need to drop two more lines as below:
The lines intersect the enthalpy scale at 107.5 and 175.5 (approximately). Once again, we take the difference of these two numbers, 175.5 – 107.5 = 68 Btu per pound.
68 Btu/lb is the “net refrigeration effect” for our system, or, the amount of cooling capacity per pound of refrigerant flowing.
To find the mass flow of refrigerant, in pounds per minute, we divide 400 (Btu per minute of cooling for the total system) by 68 Btu/min (net refrigeration effect).
400 / 68 = 5.88 pounds per minute is being circulated to achieve the 2 tons of cooling in this system.
Now that we have mass flow, 6.88 lb/min, enthalpy of compression, 11 Btu/lb, we can work the first equation:
(6.88 X 11) / 42.43 = 1.78 horse power.
Knowing that our system should use 1.78 horse power, we can now determine if the unit is overloaded, under loaded, drawing too high of current, etc.
I worked for over thirty years in the HVACR industry. I have designed, installed, serviced, and trouble shot units of various types throughout the years. The posts here are information based on that experience, I hope you find them useful. If you have a different experience, please comment.