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Pressure Enthalpy Charts

A “good” service technician can find the usual problem – it takes a “very good” service technician to find the unusual!  Knowing how to construct and interpret a pressure enthalpy (PH chart separates the good technician from the very good service engineer.

Figure 1 is a PH diagram for R-22 refrigerant.  These PH diagrams are available for all the common refrigerant gasses.

Figure 1.

Note the parabolic curve.  It goes up at an angle (labeled “Saturated Liquid oF), to the right, makes a loop, and comes back down (labeled “Saturated Vapor oF), curving to the left.  The numbers on the right side of the loop are the same temperatures on the left side of the loop.  This illustrates that when the refrigerant has gone from one side of the loop to the other side of the loop, all of the refrigerant has become a gas, or a liquid, and is still the same temperature it was at the beginning.

Figure 2.

Figure 2 is a graphic representation of what is happing in an operating refrigeration system, superimposed on a PH chart.  The shaded area is liquid refrigerant.  The bubbles represent refrigerant vapor.

Figure 3.

Refer to Figure 3.  Point A is where refrigerant enters the compressor.  Line A-B is the compression stroke.  Point A is suction pressure; point B is the head pressure.  Line B-C represents de-super-heating of the refrigerant gas, done in part of the condenser.  By point D, there should be 100% liquid refrigerant condensed entering the liquid line.  Points D to E represent the liquid line and all its components.  Point E to F is the evaporator.  Point F can be considered the evaporator outlet.  However, point F to A is all the superheat from the evaporator and suction line.

In order to plot a PH chart, we need to take only two pressures and four temperatures.  See Figure 4.  The temperature (T) and pressure (P) measurements must be accurate.  For temperatures, use a good electronic pyrometer, clean the pipe of dirt and oil where measurements are taken, make sure the probe is in tight contact with the pipe, and insulate the probe from ambient effect.  The gauges used for pressures should be accurate.

Figure 4.

The system should be running at least 10 minutes to stabilize before measurements are taken and plotted.  Label and record all readings for entering on the PH chart after all readings are taken.

T2 should be measured about 6 to 12 inches away from the compressors.  P1 should be taken at the suction service valve.  If there is no suction service valve on the compressor, suction pressure can be taken as close to the compressor as possible on any existing service port in the suction line, or a “tap” valve will have to be installed on the suction line as close to the compressor as possible.

T1 and P2 are the points where the discharge gas leaves the compressor.  Measure T2, 6 to 12 inches away from the compressor.  P2 is discharge pressure, commonly called head pressure.  Measure the pressure at the discharge service valve.  If there is no valve, one will have to be provided, as for P1.  (The use of braze taps is highly recommended to prevent leaks, especially on the discharge side.)

T3 is the point where the refrigerant leaves the condenser.

T4 is the point of entrance to the expansion device, a TXV, cap tube, orifice fitting, etc.  This point is not on the PH chart, but as we will see, can be useful in calculating the pressure drop in a liquid line.

T1 and P1 are point A, compressor inlet.  (See Figures 2 and 3.)  T2 and P2 are point B, T3 is point D.  (Point E cannot actually be identified in the field.  It will be the intersection of suction pressure, A-E, and all of the liquid line with all its accessories, D-E.)

Enter all the pressures and temperatures taken on the PH chart.

As an example:  Air conditioner R-22

T1 = 55oF, P1 = 69 Lbs., psig

T2 = 170oF, P2 = 260 Lbs., psig

T3 = 110oF

T4 = 95oF (this point is not on the PH chart)

Figure 5 shows the points plotted.  Point A is the intersection of T1 and P1.  Point B is the intersection of T2 and P2.  Point D is the intersection of the T3 saturated liquid temperature on the 260 Lb. head pressure line, P2.

Figure 5.

Figure 6 shows the calculations that can be done, right on the PH chart to diagnose a problem or problems.

Figure 6.

The chart shows 69 Lbs. as 40oF for R-22.  Therefore, the superheat is 15oF.  T1, 55oF – 40oF = 15oF.  Superheat shows on the chart as 15oF.  It is the F-A line (see Figure 3).

P2, 260 Lbs., is 120oF.  T3 is 110oF.  Therefore, there is 10oF subcooling.  The T3 and T4 readings are converted to saturation pressure to determine the pressure drop in the liquid line.  T3, 100oF = 226 Lbs.  T4, 95oF = 182 Lbs. 226 Lbs. – 182 Lbs. = 44 Lbs.

Now that we have plotted a PH chart, how do you interpret it?

The PH chart can tell us where in a system a problem exists, and thereby lead us to fix any problem(s) to get the system back to running normally.  Therefore, we have to know what is “normal” for the system.

Plot a PH chart for the system as it should be working.  Connect the points with a highlighter pen.  Then plot the pressures and temperatures as actually taken on the same chart.  Use a different colored pen to connect the points.  The problem should be very obvious when you compare the two circuits.

If available, always use manufacturers specifications to plot the “normal” chart.  If those specifications are not available, a chart can still be plotted using approximate design criteria as shown in Figure 7.

Figure 7.

Using the example of the R-22 air conditioner we plotted, we found converting T3 and T4 to pressures resulted in a 44 Lb. pressure drop in the liquid line.  This is far too much pressure drop.  High-pressure drop in a liquid line reduces the available pressure at the inlet to an expansion device, and therefore may affect its capacity.  High-pressure drop will cause flash gas, affecting the capacity of an expansion device.  It is generally considered good practice to limit R-22 air conditioning liquid line pressure drop to 8 to 14 Lbs. maximum.

Probably, the complaint about the example system was “not enough cooling.”  The high-pressure drop means there is a restriction in the liquid line.  A drier is getting plugged; the liquid line is under-sized; the line has been kinked; etc.  We now know where to look, and what to look for.

For another example of using the PH chart, let’s assume we’ve been called to a trouble job involving a walk-in freezer.  The freezer is supposed to hold a box temperature of –10°F.  The system worked fine for the past two years, but now the box cannot pull down below 0°F.  Product loading has not changed.  The system is a field build-up R-502 system.  Using Figure 7 as design criteria, first construct a normal PH chart for a R-502 low temperature system.  The design criteria would be a suction temperature of –20°F, condensing temperature 100°F, and superheat 20°F.  Converting temperatures to pressures for R-502:  -20°F = 15 Lbs., 100°F = 216 Lbs.

Begin by plotting point F.  Point F is the saturation temperature of the suction side before any superheat is added.  It is, therefore, the intersection of the –20°F lines.  Point A is plotted by adding the superheat to point F.  Line F-A then, represents all the superheat added in the evaporator and suction line; 20°F.  This results in point A being the intersection of the –20°F line and the 0°F line.

Discharge superheat consists of suction superheat, heat of compression, compressor friction, and motor heat, all added together.  As you can see, it is impossible to accurately predict discharge superheat in the field.  An added 40°F to 80°F is generally accepted temperature range for discharge superheat. 

Caution:        Discharge temperature should never exceed 212°F!   

Tech Tip:       A frequently used rule of thumb for finding discharge superheat is:


Ambient Temperature + 100°F = Discharge Temperature   

In our example, we will plot point B using 80°F discharge superheat.  Consequently, 100°F saturated discharge temperature +80°F = 180°F.

Plot point B at the intersection of the 100°F line and 180°F line.  Line B-C now represents the de-superheating of the refrigerant in the condenser.  Point D is the outlet of the condenser, the intersection of the 100°F line and the “saturated liquid °F” line.  No sub-cooling is shown on the representative PH chart, unless it is specifically known.  (Most systems with a receiver have little or no sub-cooling.  The efficiency of a system with a receiver never increases by getting subcooling at the expense of higher head pressure.)

As has been previously stated, point E cannot be actually found in the field.  In order to complete our chart and plot point E, simply drop a line straight down from point D to intersect the suction temperature/pressure line, in this case, the –20°F or 15-psig line.  Now connect the points with ah highlighter pen.  This PH chart, Figure 8, can now be used for comparison to what will actually be found when pressure and temperature readings are taken and plotted on the chart.

Figure 8.

Refer back to Figure 4.  We need to take only two pressures and four temperatures to plot the actual conditions chart.  For our example, we find the following:

P1 = 15 psig

P2 = 216 psig

T1 = 20°F

T2 = 205°F

T3 = 100°F

T4 = 98°F

See Figure 9.

Figure 9.

After plotting the points, connect them with a different color highlighter than used for the “normal” PH chart.  (In the example, we have used a dotted line to show the discrepancy.)  It will be very apparent where the problem is.  In our example, it is too much suction superheat, 40°F.  T3 and T4 show only a 2°F temperature drop, or 6 Lb. DP.  See Figure 10.

Figure 10.

On this system, the focus would most likely be on the TXV, partially plugged inlet strainer, bad power element, grossly misadjusted, etc.  A poorly insulated suction line running through a warm space could also cause high suction superheat.  In any event, we now know what is wrong and where to concentrate our efforts to correct the problem.  Too high superheat cuts capacity and would result in the box temperature being elevated.

By making a PH chart of a normal system and overlaying the chart with the actual found conditions, diagnostic time can often be shortened for problem jobs.

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