Eugene Silberstein teaches his class based on his book, "Pressure Enthalpy Without Tears," at the 2023 HVACR Training Symposium at the Kalos HQ in Clermont, FL. Eugene is the Director of Technical Education and Standards at ESCO and is a current co-author of the RACT manual.
Many people are intimidated upon their first exposure to pressure enthalpy, and there are barriers to understanding that prevent us from troubleshooting systems. It's helpful to think of a pressure enthalpy diagram as a picture that represents an entire system and provides value to technicians. When you plot a pressure enthalpy diagram, you can get an idea as to whether a system is functioning as efficiently as designed.
A pressure enthalpy chart can tell you about the system's net refrigeration effect (NRE), total heat of rejection (THOR), heat of compression (HOC), coefficient of performance (COP), mass flow rate per ton (MFR/ton), system mass flow rate (MFR), compression ratio, theoretical horsepower per ton (THP/ton), Energy Efficiency Ratio (EER & EER2), Seasonal Energy Efficiency Ratio (SEER & SEER2), evaporator and condenser capacity (in BTUs/hour), and compressor volumetric efficiency (in CFM).
To plot a system on a pressure enthalpy chart, you need to know the high-side pressure, low-side pressure, condenser outlet temperature, evaporator outlet temperature, and compressor inlet temperature. You're already picking up many of these readings when you measure superheat and subcooling, which you already do during a typical service or maintenance procedure.
The vertical axis of a pressure enthalpy chart shows the pressure (in PSIA, not PSIG); therefore, horizontal lines represent constant pressure. If you are using gauge pressure, you will need to add 14.7 to your numbers to get the PSIA. The horizontal axis represents enthalpy, and the vertical lines on a pressure enthalpy chart represent constant enthalpy; enthalpy is a measure of the total heat content.
The saturation curve or "thumbprint curve" on the pressure enthalpy chart represents the values on your P-T chart; the refrigerant in that range is a mix of liquid and vapor; anything to the left of the curve represents subcooled liquid, and anything to the right is superheated vapor. If a point is closer to the left edge of the saturation curve, it is mostly liquid but still a liquid-vapor mixture; points closer to the right edge of the curve are mostly vapor but are still at saturation. To the right of the curve, the lines that bend toward the x-axis outside of the saturation curve represent lines of constant temperature; other curved lines that trend slightly upward are lines of constant volume, and the more steeply upward-curved lines represent lines of constant entropy.
A completed chart contains a parallelogram that represents the system. The compressor is represented by a diagonal line (of constant entropy) trending up and to the right. Typically, a horizontal line on top will represent the condenser, and a horizontal line on the bottom will represent the evaporator. A vertical line connecting the horizontal lines typically represents the metering device; the heat content stays the same, but the pressure and temperature change. The position of the parallelogram will indicate potential problems with the refrigerant charge. (Overcharged systems are up and to the left, and undercharged systems are down and to the right.) The shape and size of the parallelogram can also indicate airflow or metering device problems.
When we use a pressure enthalpy chart to think about efficiency, we can think of the input-to-output ratio. High outputs from low inputs indicate higher efficiencies, whereas low outputs from high inputs indicate lower efficiencies. The coefficient of performance is an indicator of efficiency and is related to EER and SEER, and we can use the net refrigeration effect and heat of compression to think about performance and cost, respectively.
To plot a system, start by drawing a horizontal line through the point with the condenser saturation temperature. Then, do the same for the evaporator saturation temperature. Locate the condenser outlet temperature right outside the curve and draw a vertical line that intersects both horizontal lines. Then, plot the evaporator outlet temperature and compressor inlet temperatures; use them to draw a diagonal line along a line of constant entropy.
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