How to Draw a Refrigeration Cycle (1)
When you can draw a refrigeration cycle on a p-h diagram, you can easily obtain the refrigerant characteristics through the diagram. Let's study about "How to draw a refrigerant cycle" in series, using the documents with courtesy of Mr. Yoshihiro, UDAGAWA, Toshiba-Carrier Corporation.
A p-h diagram for R22 in SI unit is used.

The diagram is attached to the annex at the end of "Examination Textbook for The First and Second Class in SI unit" or "Examination Textbook for the Third Class in SI units" published by JSRAE.

1. What is a p-h diagram?
A p-h diagram is a figure with a vertical axis of absolute pressure and a horizontal axis of specific enthalpy. It is an important diagram used frequently for a performance calculation of a refrigerating machine.

A p-h diagram is made respectively for a specified refrigerant. It can, of course, not be used for another refrigerant. Diagrams are made respectively for conventional unit and SI unit
A p-h diagram has a lot of thin lines, whose names and natures are important.

p-h diagram

For example, an iso-specific volume line is a line on which the specific volume has the same value.

The following example is a simplified p-h diagram on which a refrigeration cycle is drawn.
Point 1 to 4 show the following.

Point 1 to 2: Refrigerant change in a compressor
Point 2 to 3: Refrigerant change in a condenser
Point 3 to 4: Refrigerant change through an expansion valve
Point 4 to 1: Refrigerant change in an evaporator

2. How to draw a refrigeration cycle on a p-h diagram

(Question)

Draw the following refrigeration cycle on a p-h diagram.
Refrigerant
Evaporating Temperature
Condensing Temperature
Liquid Temperature before the Expansion Valve
Suction Gas Temperature
R22
-15°C
30°C
25°C
-10°C

Notice : Conditions may be given as follows. Meanings of both are the same.
Refrigerant
Evaporating Pressure
Condensing Pressure
Subcool
Superheat
R22
0.296 MPa
1.192 MPa
5°C
5°C

(Procedure for an Answer)
(1) Draw a line for a condenser
Draw a horizontal line (1) crossing both on a saturated vapor line and saturated liquid line at 30°C. The length of the line is adjusted later. Shorter the line is, easier adjusted.
A horizontal line passing through a condensing pressure of 1.192 MPa for 30 °C is same as the above line.
(2) Draw a line for an evaporator
Draw a horizontal line (2) crossing both on a saturated vapor line and saturated liquid line at -15°C. The length of the line is also adjusted later. A horizontal line passing through a condensing pressure of 0.296 MPa for -15 °C is same as the above line.
(3) Draw a line for a compressor
An intersection point (Point 1) of the line for an evaporator (2) and an isothermal line for -10°C shows a position of a suction gas to a compressor. Draw a curve (3) from Point 1 parallel to an isentropic line. An isentropic line is a curve.

(4) Draw a line through an expansion valve
Since the refrigerant is a subcooled liquid of 25°C, draw a vertical line (4) that passes through a saturated liquid line for 25°C (Point E). An intersection point (Point 3) of the line (4) and the line (1) shows a position before the expansion valve. Point E obtained from a saturate pressure of 1.045 MPa for 25°C is the same.

(5) Complete the figure
You can draw a refrigeration cycle on a A p-h diagram as above stated. Since it is not good to look at, please finish it like the following figure by erasing extra lines outside of Point 1 to 4. You can read various values from the diagram.

Have you finished drawing a refrigeration cycle on a p-h diagram well?
You can read out a specific volume (v1) and specific enthalpies (h1, h2, h4) from the figure.

(Answer)
A specific volume and specific enthalpies at each point
Specific volume v1=0.08 m3/kg、 Specific enthalpy h1=403 kJ/kg, h2=438 kJ/kg, h4=230 kJ/kg

You can calculate a performance of a refrigerating machine by using these values.

In the next round of this series, let's study about "Changes of refrigerant conditions". Conditions of a refrigerant are changed by turns in a refrigeration cycle. A typical one will be taken up.
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