Pressure / Enthalpy Diagram Example

This article was written by my buddy and Canadian Supertech Tim Tanguay. Thanks, Tim!


This P/E chart shows R410a at 100°F saturated condensing temp, 10°F SC, 40°F saturated suction temp, 20°F SH at the compressor.
The green highlighted thumb shape is the saturation zone. Everything that occurs in the saturation zone is a latent (change of state) process.
Everything that occurs to the right (superheating) and left (subcooling) is a sensible process.
Go to the movies in your mind. Imagine that you are one pound of 410A. We commence our journey at the rightmost point on the upper orange highlighted line.
At this point, you have just left the compressor. You are a superheated vapor, with a temperature of 145°F. You enter the condenser and start rejecting heat to the atmosphere. After rejecting 45°F of sensible heat (desuperheating), you hit the saturated condensing zone (100°F), and you turn into a drop of liquid. As you march your way through the condenser (follow the line left), you reject latent energy but stay at the same temperature. As your latent energy decreases, you become more liquid until, finally, you are a solid column of liquid, and you exit the saturation zone to the left of the thumb. You then give up another 10°F of sensible heat to the air and become a 90°F subcooled liquid.
You approach the sight glass as a 90°F subcooled liquid under approximately 350 PSIG of pressure. As you pass the sight glass, you fart a few bubbles just to mess with the refrigeration mechanic observing the process. You squeeze your way through the tiny orifice in the metering device and emerge into the evaporator, solidly back into the saturation zone. You find yourself as a 40°F saturated liquid at 125 PSIG (approx 78% liquid, 22% vapor, indicated along the constant quality lines).
Now you make your way along the bottom line towards the right side of the thumb. You absorb heat energy from the warm return air rushing over the copper and aluminum evaporator fins. The heat you absorb boils you dry. You are naught but a vapor, and as such, the energy from the return air increases your sensible heat by 10°F. You emerge from the evaporator as a 50°F superheated vapor. As your journey progresses
towards the suction inlet of the compressor, you pick up another 10°F of sensible heat.
You enter the suction port of the compressor as a 60°F, superheated vapor. The compressor puts you through a strenuous workout, squeezes you into a smaller volume, and increases your temperature by about 85°F in the process.
You emerge as a superheated 145°F vapor. The process begins anew.
There are a few things to look at:
The numbers on the top represent enthalpy energy, as BTUs per pound. In this particular example, the sensible portions of the condenser account for approx 20% (eyeball estimate) of the total heat rejected in the condenser. The other 80% of the process is latent.
On the right-hand side of the PE diagram, you have a specific volume, which is represented as curved dotted lines. As SST decreases, the specific volume increases and vapor density decreases. This fact alone is why refrigeration compressors need to be physically larger. As specific volume increases, the volumetric efficiency of compressors decrease. Lower SST's (suction saturation temp) require larger compressor displacement because they need to move more gas to obtain the required mass flow. In A/C and refrigeration, the mass flow of refrigerant through the system ultimately determines your system capacity.
At 40°F, the latent heat of vaporization of 410A is approx 75 BTU/LB. Compare that to water, which has a latent heat of vaporization of approx 970 BTU'S per pound at 212°F/14.69 PSIA, and you begin to realize why dehydration of a system takes so darn long. It takes a LOT of energy to boil water off.
In the evaporator, about 10% of the process is sensible. That is why a unit that is short on refrigerant isn't able to cool properly. The refrigerant boils off, leaving a large portion of the coil to collect sensible heat (higher superheat). The amount of heat that sensible processes remove from the air stream is relatively tiny; thus, we lose capacity.
So, too, with things like water. The sensible heats involved with changing temperature are minuscule when compared to the amount of heat required to change state. Universally, latent changes require orders of magnitude more energy than sensible changes.
—Tim

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