A doctor always checks the blood pressure and body temperature as well as pulse rate to assess the body system. Similarly, pressure and temperature indicate the healthiness of a refrigeration system. Pressure changes the boiling point of a liquid. The temperature of a vapour or a gas is related to its pressure. Therefore, we always consider the system parameters at constant pressure and temperature.
Majority of refrigerants are found in a gas form below boiling temperature at normal outside temperature _ but if we hold them in liquid state by keeping them under pressure such as in a cylinder or receiver, they are at certain nearby boiling temperature.
Status of suction and discharge pressure in a vapour compression system
Looking to the pressure temperature chart of R-12, we read at 00C (320F) the pressure is 2.1 Kg/cm2 this indicates that liquid R-12 will boil at temperature 0°C (32°F) if pressure on surface is 2.1 Kg/cm2 when a refrigeration system is working with a suction pressure of 2.1 Kg/cm2. We can decide that the liquid refrigerant is boiling in the evaporator at a temperature 0°C (32°F).
Discharge of temperature
The temperature of the discharge vapour coming out of the compressor, however, is much more than the condensing temperature of 48.90 (12°F). This is because the gas gets super-heated 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 vapour can be removed to condense the vapour. The first few pipes of the condenser are utilised for desuper-heating the discharge vapour.
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 kept.
For example, a R-22 cylinder with a temperature of a 26.6°C (80°F) will show a pressure of 10.1 kg/cm²G (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 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 10.1 kg/cm²G (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 11.8 kg/cm²G (168.4 psig) the saturation pressure for R-22 at 32.2°C (90°F). The relationship of pressure and temperature of a refrigerant holds good only when some liquid is available in the container. This is called the ‘saturated condition’.
Super heat
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 11.8 kg/cm²G (168.4 psig) – the saturation pressure at 32.2°C (90°F). Any further increase in the temperature of the cylinder above 32.2°C (90°F) will only heat up the vapour inside the cylinder, as there is no liquid left to boil. So the temperature of the vapour will rise above its initial saturation temperature. The vapour is then, said to be ‘superheated’. If the temperature of the cylinder rises (so does the temperature of the vapour inside) to say, 37.8°C (100°F), the vapour is superheated by 5.6°C (10°F) from its saturation temperature of 32.2°C (90°F). The superheated vapour 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 vapour.
In a refrigeration system we have 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 vapour 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 4.4°C (40°F) with the expansion valve adjusted to maintain a superheated of 5.55°C (10°F), it means that all the liquid refrigerant boiled off before reaching the suction outlet of the evaporator and the vapour got heated up by 5.55°C (10°F) from its saturation point of 4.4°C (40°F) by the time the vapour reached the point where the bulb is mounted. The vapour temperature become 10°C (50°F) but the pressure of the vapour remained at 4.85 kg/cm²G (68.5 psig) only, since the suction pressure is kept constant by the operation of the compressor.
Therefore, 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 4.8 kg/cm²G (68.5 psig) corresponding to the saturation temperature is 4.4°C (40°F). If the suction line temperature is 100C (50°F) the superheat is 10-4.4 = 5.6°C (50-40°F = 10°F). To measure this superheat fairly accurately, thermo wells 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, say R-22, is 13.8 kg/cm²G (195.9 psig), from the tables we can find 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 13.8 kg/cm²G (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 is kept constant in the condenser by the compressor. The liquid can get sub-cooled below the saturation temperature in the condenser because the temperature of the water/air at the inlet to the condenser being 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 vapour.
As is obvious, the pre-requisite for the sub-cooling of a liquid and the superheating of a vapour is that the liquid and vapour 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 vapour. Also, the liquid can get sub-cooled at the bottom of the condenser as it is away from the point of contact with the vapour. Likewise the suction vapour gets superheated in moving away from the point of contact with the liquid in the evaporator.
Pressure and temperatures
Temperature measures average speed of motion of the molecules and it measures sensible heat. It cannot measure latent heat that causes change of state of a substance.
Since we need to measure total heat (enthalpy), the unit that is used is BTU/hr or cal/hr or watt(J/sec)
BTU measure : (1) Heat Content (2) Heat Transfer (3) Heating and Cooling Capacity (4) Heating & Cooling Load (5) Heat Content of Refrigerant (Enthalpy).
British Thermal Unit is the amount of heat required to raise the temperature of one pound of water through one degree Fahrenheit.
Specific Heat is the amount of heat, measured in BTUs required to raise the temperature of one pound of a substance through one degree Fahrenheit.
Specific heat helps us compare how easily various substances are heated. It also allows us to calculate the amount of heat transferred during sensible heating or cooling process.
Each substance has of its own unique specific heat. Substances with low specific heat are easily heated. Mercury has very low specific heat hence it is used in thermometers. Water has very high specific heat compared to other metals.
The same substance in different states has different specific heats. Water has sp.ht. as one whereas ice has sp.ht. 0.5 and steam has sp.ht. as 0.48.
The sensible hat can be calculated by following formula:
Q = S x W x (ΔT)
The latent heat of vaporisation of water is 970 BTU/ib (540Kcal/kg or 2258K J/kg), 1 Kcal = 3.968 BTU = 4.18KJ
The latent heat of fusion is 144 BTU/ib (80Kcal/kg or 333.5KJ/kg). This is the heat to be added to one pound of ice for changing from 320F ice into 320F water.
The total heat or
ENTHALPY= SENSIBLE HEAT + LATENT HEAT
1150 BTU = 180 BTU + 970 BTU
It is important to be able to graph heat and enthalpy because graphing these helps understand it better. It leads us to plot pressure enthalpy diagrams that are most important for practicing engineers.
These help in:
Trouble shooting the refrigerant side of a mechanical refrigerating system
Helps in seeing the functions of each part of the mechanical refrigeration system and how they work together in moving heat.
Helps in predicting temperatures and pressures one should find at various places within a system.
The BOILING TEMPERATURE is called SATURATION TEMPERATURE in mechanical refrigeration work.
Normally boiling point for any substance is at atmospheric pressure. Hence it can be stated that every fluid has one boiling point but many saturation temperatures.
The major work in refrigeration is carried out by latent heat. The refrigerant is changed from a liquid into a gas in evaporator, where heat is absorbed from surrounding medium. Likewise; the refrigerant is changed from a gas back into a liquid in the condenser coil, where heat is rejected to outdoor.
The fluid undergoes sensible heating, then latent heating and one saturated then sensible heating known as superheating starts for vapour.
Refrigerant normally enters evaporator as sub cooled liquid. As with water this means it is below its saturation temperature of 212°F (100°C) at sea level, and is therefore subcooled. Further even water we drink is subcooled in technical terms.
When heat is added to liquid refrigerant in evaporator it EVAPORATES and when heat is removed in condenser it CONDESES.
A refrigerant in almost any condition can be found in a system at any moment of time : Sub-cooled liquid, Saturated liquid, Saturated liquid/vapour mixture, Saturated vapour, or Super-heated gas. All are present at a time at different locations in a system. This happens because the function and pressure in each component is different.
Changing pressure in the system can alter the saturation pressure and temperature. The same system absorbs heat and evaporates liquid refrigerant into a gas at 400F (50C) can also reject heat and condense in to liquid at 120°F (45°C) in condenser. That is why evaporator and condenser operate at different pressures in the system.
Pressure
Pressure is defined as force per unit area. Pressure behaves differently for solids and liquids or gases. It tends to exert only in one direction for solids. Fluids on the other hand tend to exert pressure equally in all directions.
We are used to live in the fluid pressure on our bodies by earth’s atmosphere. It exerts 14.7 pounds per sq. in (1.033 kg/cm2) or (101.325 kPPa or 1.01325 bar) pressure all over our body at sea level. This pressure is called atmospheric pressure. This pressure can be measured by barometer and hence it is called barometric pressure.
The depth of atmosphere at sea level is 60 miles and we live at the bottom of it. As we go higher there is less atmosphere over us as compared to sea level. In conventional airliner, the cabin must be pressurized to avoid effects of extreme pressure changes caused by altitude.
The drier air is denser than moist air. The colder air is denser than warm air. The barometric pressure is therefore highest on cold, dry days. In refrigeration systems, pressure exists in 3 ranges, above atmospheric, at, or below atmospheric pressures. Efforts are made to select the refrigerant, which normally would operate above atmospheric pressure for the particular application.
The system for measurement of pressure as absolute and gauge pressures. Absolute pressure is used for weather reporting and forecasting as well as for engineering calculations and for plotting pressure/enthalpy charts. Gauge pressure is used for all services work. This can be measured with the help of pressure gauge either directly installed on a plant or connected when needed. Gauge pressure equal or below atmospheric pressure is expressed as inches of mercury vacuum. Gauge pressure equal to or above atmospheric pressure is expressed as PSIG.
Gauge pressure + Atmospheric pressure = Absolute Pressure.
Normally a compound gauge is used on suction side as it has markings for above or below atmospheric pressure. The gauges are also marked with corresponding saturation temperatures for common refrigerants for ease of service technicians.
The pressure of a refrigerant and its saturation temperature are closely related, and we need only know one to find out other. Saturation temperature is really a boiling point of refrigerant at that pressure.
As the pressure increases, the boiling point increases and vice versa. Even though refrigeration pressure can be used to find saturation temperature, these facts do not guarantee that the refrigerant is at saturated conditions. From the temperature /enthalpy charts it can be seen that, at any single pressure, the refrigerant can exist as a sub cooled liquid, a saturated vapour or a superheated gas. If the liquid and gas states of the refrigerant are both present in one place, the refrigerant is at its saturation temperature. If liquid is present, it may be at the saturation temperature or may be below the saturation temperature (sub-cooled liquid). A temperature reading will be needed in addition to determine its condition. The same applies for gas.
In a refrigerant cylinder, if liquid and gas both are present, then cylinder pressure will be corresponding to atmospheric temperature if it has been stored for longer duration for conditions to stabilize. In winter the cylinder pressure will be less and it would be difficult to charge gas in the system.
