Pressure Enthalpy chart 

Pressure Enthalpy Chart or P E Chart is considered the 2nd most important chart in the HVAC industry, after the Psychrometric chart, but what in the world is the Enthalpy? Well without going into much really boring details all what you need to know is that “the enthalpy of any object is the total amount of heat that object contain”. The word enthalpy is based on the Greek noun enthalpos (ἔνθαλπος), which means heating.

In order to understand the Pressure Enthalpy Chart fully, we are going to do an experiment, let's assume you have jar full of water.

pressure enthalpy experiment

Status (A)

The Jar is open from the top (the pressure is atmospheric) and it is at normal room temperature,

  • (Pressure) P1=14.7 PSI
  • (Temperature) T1=78 °F
  • (Enthalpy) H1= 0 Btu/Lb.

Note: assuming enthalpy as the base line or  Zero at the beginning of  the experiment.

Status (B)

The Jar still open (pressure is atmospheric) but we are heating the Jar hence increasing the Temperature and the Enthalpy,

  • P2=14.7 PSI         (still the pressure is atmospheric)
  • T2=100 °F           (the temperature rises)
  •  H2= 20 Btu/Lb.   (no change in heat)


Status (C)

We cover the jar and then start vacuuming, but we don’t heat any more (do you remember the pressure- temperature relationship from the thermal theory page), yes you are correct  if  you lower the pressure the temperature  will also decrease, without adding or removing any heat.

So in this case we are lowering the pressure by vacuuming thus the temperature will also drop without dropping the enthalpy (The heat)

  • P3=5 PSI              (the pressure drops due to vacuum)
  • T3=78 °F              (the temperature drops as well)
  •  H3= 20 Btu/Lb.     (no change in heat)
pressure enthalpy experiment result

Let s take a closer look at A and C, the only thing that really happen, is that we have added heat from A to C, the temperature is constant and the pressure has dropped.

So from this experiment it is safe to say “in Order to add heat to a system and to maintain its Temperature Constant you must Drop the Pressure.

So the 1st hypothetical experiment will lead us to the next one, now assume you have a system (still the Jar and the water that in thermodynamics we call it a thermal system) and that system is at Constant Temperature let’s say T=60 F  so we are adding heat to that system and we are recording how much heat we are adding and we also recording how much pressure it will drop, to maintain the same Constant temperature so it’s very important to note that the temperature at any given time is constant or the same (T=60 F).

The X-axis represents the Enthalpy (or the heat we add) that we are increasing and the Y-axis represent the pressure that is dropping to maintain the constant temperature, we will plot or record that, and that’s is the Pressure Enthalpy chart.

pressure enthalpy chart detailed

Let’s take a closer look at the graph of the Pressure Enthalpy Chart we just did, and let’s try to understand it together, 1st we could divide the graph into 3 different sections (A, B, C)

Section A

We are adding Heat (increasing the enthalpy) to the water (please remember the water still in liquid form) the pressure is dropping (blue line). Temperature is the same  T=60 F

Question!  do we consider this heat to be Sensible or Latent heat ? Well it is very good question, it is sensible because even so the temperature is constant but the pressure is dropping so we can still sense it.

Section B

Great all fine and dandy so far. Till we reach the “Saturated Liquid point “ this is the point that the water cannot take any more heat, any extra heat we will add to the water, it will start to evaporate.

Of course the heat we add in section B “red line”  it causes the water to evaporate thus it must be latent and that explains why the pressure stays constant while the temperature is already constant  so we cannot sense that heat so it must be latent. Okay great now what?

Now the water in the transition state meaning it is transforming into gas, till we finally reach the Saturated Gas point. That mean at this point the water is fully 100% in gaseous state there is no liquid left, if we remove any amount of heat from it  the water will condensate again/

Section C

Now the water is 100% gas, but we are still adding heat, remember the temperature  still constant thus the pressure must  continue to drop just like in section A and that heat is?  Yes you said it is sensible heat of course 

It should be noted that if the water didn’t reach the "liquid saturated point" we call it sub-cooled liquid and it doesn't mean it is cold ,  it means  that it has more room to take more heat without evaporating.

And the water vapor after the "gas saturated point" we call it super-heated vapor. Again it doesn't mean it is hot, it means that it can lose heat without condensing.

Wow that was a lot... wasn't it?

Okay I promise there is not much left in this topic.

Let's repeat the same experiment (the Pressure Enthalpy Chart experiment  numerous times and each time we do it at different temperatures (10 F, 20 F, 30 F, 40 F, & 50 F) and we will plot different curves and lines.

pressure enthalpy chart

Once we are done with Pressure Enthalpy Chart graph. We will discover some interesting points, If we connect all the liquid and gas saturated points at all of the different temperatures lines we will construct what we call the  saturated curve, anything to the left of that curve is sub-cooled liquid and anything to the right of that curve is Super-heated gas, all the point on the curve itself  is saturated-regardless whether gas or liquid.

From left to right at the same pressure line inside the Curve, the Pressure and temperature are constant.

Inside the curve of the Pressure Enthalpy Chart each pressure correspond to single temperature what we call the boiling point at that specific pressure.

Example: the boiling point of water at atmospheric pressure 14.7 PSI is 212 F, if we increase the pressure the point will also increase, that relationship between the saturated temperature and saturated pressure, construct the 3rd most important chart in the industry ( P-T chart ) or the Pressure–Temperature chart.  

The tip of the saturation curve is what we call the flashing point, it is at that pressure the substance (in our case is water) will go from liquid to gas immediately without transition we call that the flashing pressure.

Each Refrigerant (Freon) has its own Pressure Enthalpy chart, and based on that we can determine what the best use of that Refrigerant is.

For example the Boiling point of R22    (under 75 PSI)  is around 45 F that make R-22 not that suitable for freezers but it will be perfect for Air conditioners or a  Heat pump

in the other hand R-502 the boiling point under the same pressure is about 35 F that’s what makes it excellent for freezers. And so on.

Well that was a very heavy topic aren’t you glad we got it over with? So let’s move forward to the refrigeration cycle so we can see how it’s all come together. But before we go there, here is some Real diagram for Pressure Enthalpy Chart

Freon R22 pressure enthalpy chart

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