Pressure-Temperature Relation, Superheat and Sub-cooling
Page Summary
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We have seen that as pressure changes, the boiling point of a liquid or condensing temperature of the vapor/gas also changes. We have also seen as a corollary from the example of water in the fundamental concepts and terminology section under that for every pressure exerted on the surface of the liquid, there is a corresponding boiling point for the liquid (or condensing temperature for the gas).
Boiling points of the majority of refrigerants are below normal outside temperature; so they exist as gas. They are held in a liquid state by keeping them under pressure, such as in a cylinder, receiver, etc. |
Refrigerant Tables
The thermodynamic properties of refrigerants are given in the tables appearing in the appendix pages. These tables give the following information.
- Boiling temperature and corresponding pressures in absolute pressure or gauge pressure.
- Volume (in m^3/kg or ft.^3/lb.) and density (in kg/m^3 or lb./ft.^3) for liquid and vapor (under saturated conditions).
- Enthalpy (amount of heat contained) of liquid and vapor, in kcal/kg or BTU per pound. The difference between the enthalpies of vapor and liquid gives the latent heat of vaporization.
Significance of Suction and Discharge Pressures in a Vapor-Compression System
From the pressure-temperature chart of R-134a, read against 0 *C (32 *F) pressure of 27.79 psig. This means that liquid R-134a will boil at a temperature of 0 *C (32 *F), if the pressure on the liquid surface is 27.79 psig. So when an R-134a refrigeration system is working with a suction pressure of 27.79 psig, the liquid refrigerant is boiling in the evaporator at 0 *C (32 *F), as suction pressure is almost equal to the pressure existing in the evaporator (neglecting the pressure drop in the suction line). The table of R-134a, shows the pressure corresponding to 48.9 *C (120 *F) as 171.14 psig (1179.96 kPa), i.e. liquid R-134a will boil at 48.9 *C (120 *F) if its pressure is 171.14 psig. This also means gaseous R-134a at a pressure of 171.14 psig can condense into a liquid, if it is cooled below its corresponding (saturation) temperature of 48.9 *C (120 *F). So when an R-134a system is working with a discharge pressure of 171.14 psig, it means that gaseous R-134a is condensing at 48.9 *C (120 *F) in the condenser.
Illustrating Suction and Discharge Pressures and Discharge Temperatures
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Discharge TemperatureThe temperature of the discharge vapor coming out of the compressor, however, is much more than the condensing temperature of 48.9 *C (120 *F).This is because the gas gets superheated while on its way from the evaporator to the compressor. It also gets superheated in the compressor during compression. In the condenser, this superheat will have to be removed first, before the latent heat from the vapor can be removed to condense the vapor. The first few pipes of the condenser are utilized for de-superheating the discharge vapor.
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Saturated Condition
In a closed container, such as a cylinder, if a quantity of refrigerant is available in the liquid form, a pressure gauge connected to the cylinder will show a pressure corresponding to the temperature of the liquid. This temperature will be the same as the temperature of the room in which the cylinder is located, for example, an R-22 cylinder with a temperature of 26.6 *C (80 *F) will show a pressure of 143.63 psig. If the temperature of the room (or the cylinder) goes up, to say 32.2 *C (90 *F), it will be noticed that the pressure, as indicated on the gauge, will also go up. The reasons for the increase in pressure is that as the temperature goes up, some of the R-22 liquid in the cylinder is boiled or vaporized because the pressure on the surface before the temperature was raised was only 143.63 psig, which corresponds to a boiling temperature of 26.7 *C (80 *F). The boiling, however, will cease as soon as the pressure reaches 168.4 psig, which is the saturation pressure for R-22 at 32.2 *C (90 *F).
The relationship between the pressure and temperature of a refrigerant holds good only when some liquid is available in the container. This is called the saturated condition.
The relationship between the pressure and temperature of a refrigerant holds good only when some liquid is available in the container. This is called the saturated condition.
Superheat
Let us assume that the cylinder in our example had only a very small quantity of R-22 liquid and that even the last drop of liquid had boiled off, just when the temperature touched 32.2 *C (90 *F). At that time, the pressure would have been 168.4 psig - the saturation pressure at 32.2 *C. Any further increase in the temperature of the cylinder above 32.2 *C (90 *F) will only heat up the vapor inside the cylinder, as there is no liquid left to boil. So the temperature of the vapor will rise above its initial saturation temperature. The vapor is then said to be superheated. If the temperature of the cylinder rises, so does the temperature of the vapor inside, to say, 37.8 *C (100 *F), the vapor is superheated by 5.6 *C (10 *F) from its saturation temperature of 32.2 *C (90 *F). The superheated vapor will obey the gas law. So there will be an increase in pressure on superheating; but the increase will be very small, compared to the increase, had it been a saturated vapor.
In a refrigeration system, we have a saturated condition in the evaporator and liquid receiver (or the bottom portion of the condenser) and a superheated condition in the suction and discharge lines. When we say that an expansion valve is adjusted for a superheat of 5.55 *C (10 *F), we mean that in the suction line at the place where the expansion valve bulb is mounted, there is no liquid at all; but that the vapor at this place also is superheated by 5.55 *C (10 *F) above its saturation point. If the refrigerant in the evaporator is boiling at a temperature of 4.44 *C (40 *F) with the expansion valve adjusted to maintain a superheat of 5.55 *C (10 *F), it means that all the liquid refrigerant has boiled off before reaching the suction outlet of the evaporator, and the vapor got heated up by 5.55 *C (10 *F) from the saturation point of 4.44 *C (40 *F) by the time the vapor reached the point where the bulb was mounted. The vapor temperature because 10 *C (50 *F) but the pressure of the vapor remained at 68.5 psig, since the suction pressure is kept constant by the operation of the compressor.
In a refrigeration system, we have a saturated condition in the evaporator and liquid receiver (or the bottom portion of the condenser) and a superheated condition in the suction and discharge lines. When we say that an expansion valve is adjusted for a superheat of 5.55 *C (10 *F), we mean that in the suction line at the place where the expansion valve bulb is mounted, there is no liquid at all; but that the vapor at this place also is superheated by 5.55 *C (10 *F) above its saturation point. If the refrigerant in the evaporator is boiling at a temperature of 4.44 *C (40 *F) with the expansion valve adjusted to maintain a superheat of 5.55 *C (10 *F), it means that all the liquid refrigerant has boiled off before reaching the suction outlet of the evaporator, and the vapor got heated up by 5.55 *C (10 *F) from the saturation point of 4.44 *C (40 *F) by the time the vapor reached the point where the bulb was mounted. The vapor temperature because 10 *C (50 *F) but the pressure of the vapor remained at 68.5 psig, since the suction pressure is kept constant by the operation of the compressor.
Illustration of a Refrigeration Expansion Valve, its Components and Operation.
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To find out the superheat adjustment of the expansion valve in a system, we have to find the difference between the temperature of the suction line, at the place where the expansion valve bulb is mounted, and the saturation temperature corresponding to the evaporator pressure (or approximately the suction pressure).
In our example, the suction pressure is 68.5 psig corresponding to the saturation temperature of 4.4 *C (40 *F). If the suction-line temperature is 10 *C (50 *F), the superheat is (10 - 4.4) *C = 5.6 *C. Also in Fahrenheit it is (50 - 40) *F = 10 *F. To measure this superheat fairly accurately, thermowells are provided in the suction line at the place where the expansion valve bulb is mounted. |
Sub-Cooling
If the temperature of the refrigerant liquid is less than its saturation temperature, the liquid is said to be in a sub-cooled condition. If the pressure of a liquid, i.e. R-22, is 195.9 psig, we can conclude from the tables provided in sub-page on the top, that its saturation temperature is 37.8 *C (100 *F). But if the liquid is cooled to 35 *C (95 *F) without allowing the pressure to drop down below 195.9 psig by some means, the liquid is said to be sub-cooled by 37.8 - 35 = 2.8 *C (100 - 95 = 5 *F). This condition can exist at the bottom portion of a condenser, or in the liquid line where a heat exchanger is used. The pressure will be kept constant in the condenser by the compressor operation. The liquid can get sub-cooled below its saturation temperature in the condenser, because of the of the temperature of water/air at the inlet to the condenser being comparatively low. In the liquid suction heat exchanger, the liquid gets sub-cooled below the saturation temperature, because of the cooling of the liquid line by the cold suction vapor.
Therefore, the pre-requisite for the sub-cooling of a liquid and the superheating of a vapor is that the liquid and vapor should not be in contact with each other. Liquid sub-cooling is obtained in water-cooled and air-cooled condensers which have separation arrangement between liquid and vapor. Also, the liquid can get sub-cooled at the bottom of the condenser, as it is away from the point of contact with the vapor. Similarly the suction vapor gets superheated in moving away from the point of contact with the liquid in the evaporator.
Therefore, the pre-requisite for the sub-cooling of a liquid and the superheating of a vapor is that the liquid and vapor should not be in contact with each other. Liquid sub-cooling is obtained in water-cooled and air-cooled condensers which have separation arrangement between liquid and vapor. Also, the liquid can get sub-cooled at the bottom of the condenser, as it is away from the point of contact with the vapor. Similarly the suction vapor gets superheated in moving away from the point of contact with the liquid in the evaporator.
Pressure Temperature Charts (at saturation) in IP and SI Units
Pressure-Temperature Charts - SI & IP Units