ME-803
Refrigeration & Air Conditioning
Vapor compression system : Simple Vapor compression cycle, p-h and t-s diagrams,Deviations from theoretical cycle, sub-cooling and super heating, effects of condenser and evaporator pressure on cop; multi-pressure system: removal of flash gas, multiple expansion
&compression with flash inter cooling; low temperature refrigeration: production of low temperatures, cascade system, dry ice, production of dry ice, air liquefaction system
A simple vapor compression refrigeration system consists of the following equipments: i) Compressor ii) Condenser iii) Expansion valve iv) Evaporator.
Simple vapour compression system
RAC BY:- Raju Kumar Das
Asst prof
GIIT Gwl

ME-803
Refrigeration & Air Conditioning
The schematic diagram of the arrangements as shown in Fig. The low temperature, low pressure vapor at state B is compressed by a compressor to high temperature and pressure vapor at state C. This vapor is condensed into high pressure vapor at state D in the condenser and then passes through the expansion valve. Here, the vapor is throttled down to a low pressure liquid and passed on to an evaporator, where it absorbs heat from the surroundings from the circulating fluid (being refrigerated) and vaporizes into low pressure vapor at state B. The cycle then repeats
The exchange of energy is as follows
Compressor requires work,
δ w. The work is supplied to the system from the
Surroundings. b) During condensation, heat
δ
Q
1 the equivalent of latent heat of condensation etc is lost from the refrigerator. c) During evaporation, heat
δ
Q
2 equivalent to latent heat of vaporization is absorbed by the refrigerant. d) There is no exchange of heat during throttling process through the expansion
Valve as this process occurs at constant enthalpy.
Simple Vapor Compression Cycle
RAC BY:- Raju Kumar Das
Asst prof
GIIT Gwl

ME-803
Refrigeration & Air Conditioning
Simple Vapor Compression Cycle
`Figure shows a simple vapor compression refrigeration cycle on T-s diagram for Different compression processes. The cycle works between temperatures T
1 and T
2 representing the condenser and evaporator temperatures respectively. The various process of the cycle A-B-C-D (AB’-C’-D and A-B”-C”-
D) are as given below: i) Process BC (B’-C’ or B”-C”): Isentropic compression of the vapor from state B to C. If vapor state is saturated (B),or superheated (B”), the compression iscalled dry compression. If initial state is wet (B’), the compression is called wet compression as represented by B’-C’. ii) Process CD (C’-D or C”-D): Heat rejection in condenser at constant pressure. iii) Process D-A: An irreversible adiabatic expansion of vapor through the
Expansion value. The pressure and temperature of the liquid are reduced.
The process is accompanied by portal evaporation of some liquid. The process is shown by dotted line. iv) Process A-B (AB’ or A-B”) : Heat absorption in evaporator at constant pressure. The final state depends on the quantity of heat absorbed and same may be wet (B’) dry (B) or superheated (B”).
RAC BY:- Raju Kumar Das
Asst prof
GIIT Gwl

ME-803
Refrigeration & Air Conditioning
The purpose of this article is to provide a simple explanation of these terms for those who desire a concise understanding as well asa review for those who understand the terms but want to review them. An understanding of these terms and the concepts related to them is essential to understanding the air conditioning and refrigeration mechanical – refrigerant cycle as well as being nec essary to troubleshooting cycle problems.
Most materials can exist in three forms, solids, liquids and gases. Water is a common example. Water can exist as a solid (ice), aliquid, or a gas or vapor (steam). Only a gas or vapor
(these a re interchangeable terms), can be superheated. Let’s use water as an example as we explain these terms.Water at sea level boils at 212 degrees F.
When heated to 212 degrees F the molecules which make up water are moving at a high enough speed that they overcome the air pressure above the water. As additional heat is added to liquid water at 212 degrees, the water begins to boil.
As the water boils it is changing state from a liquid to a gas. In addition, during the boiling process thetemperature remains the same (212 degrees F). There is no change in temperature during a change of state. This phenomenon is true for
RAC BY:- Raju Kumar Das
Asst prof
GIIT Gwl

ME-803
Refrigeration & Air Conditioning all substances as they change state no matter how much heat is added. As long as the water is still boiling and not all the water has completely changed to a gas (steam) the temperature remains at 212 degrees F. This means that a
Thermometer placed in boiling water will remain at 212 degrees throughout the boiling process even though heat is added to cause the water to boil. This heat of boiling is called latent heat. The word “latent” is a Latin word for “hidden”. The heat added to the water is hidden from the thermometer since the temperature remains unchanged during the boiling process. After all the water has changed to a gas or vapor (steam), then the addition of still more heat to the vaporized water or steam will cause the temperature of the steam to increase above it’s boiling temperature of 212 degrees. Any increase in temperature of the steam above it’s boiling point (212 degrees) is called “superheat”. Steam at 213 degrees F is superheated by one degree F. Superheat is then any temperature of a gas abovethe boiling point for that liquid. When a refrigerant liquid boils at a low temperature of 40 degrees in a cooling coil and then the refrigerant gas increases in temperature superheat has been added. If this refrigerant changed from a liquid to a gas or vapor at 40 degrees and then the refrigerant vapor increased in temperature to 50 degrees F, then it has been superheated by 10 degrees. We commonly think of boiling as always being accomplished by a liquid when it is hot. This is because we are familiar with boiling water. However, air conditioning and refrigeration systems use liquids (refrigerants) with much lower boiling points. If a liquid refrigerant boils at -10 degrees and is then warmedup to zero degrees, it is then a superheated gas containing 10 degrees of superheat.
Heating that same refrigerant gas to +10 degrees means that it now has been superheated by 20 degrees. Lowering the pressure over a liquid lowers the boiling point. There is less pressure above the liquid to overcome. That is why water at the top of a mountain may boil at 190 degrees (depending upon the altitude) rather than at 212 degrees F. By controlling the pressure over a liquid, we can control the boiling temperature. That is why a service technician monitors the pressures in an air conditioning system. The technician is actually monitoring the pressures and temperatures where the refrigerant is changing state.
RAC BY:- Raju Kumar Das
Asst prof
GIIT Gwl

ME-803
Refrigeration & Air Conditioning
Saturation is simply the term used to describe the point where a change of state in a substance is taking place. For water at sea level,the boiling temperature is
212 degrees F. Therefore, we say the saturation (boiling temperature) is 212 degrees. As soon as the temperature of the steam is heated above it’s“saturation” temperature, it has been superheated. Refrigerant that has boiled
(turned into a vapor) at 40 degrees has a saturation temperature of 40 degrees.
If therefrigerant vapor is heated to 41 degrees it is no longer saturated, it is then superheated by 1 degree. Remember, only a gas or vapor can be superheated.
Superheat is any temperature of a gas or vapor above it’s saturation temperature.
Sub cooling is now easy to understand. Only liquids and solids can be sub cooled. Sub cooling is any temperature of a liquid or solid below it’s saturation temperature. Let’s use water as an example again. Liquid water at sea level has a saturation (boiling) temperature of 212 degrees F. If we were to add heat to the saturated water it would first boil away with no change in temperature (remember latent heat?) and then become superheated if still more heat were added to the vapor (steam) after it had all turned to a vapor. Instead of boiling our 212 degree water by adding heat, we shall remove heat from the 212 degree water. As heat is removed from the liquid water it’s temperature will drop below it’s boiling
(saturation) temperature. Water at 211 degrees has been sub cooled by one degree F. If the temperature of the water is decreased to 180 degrees the water has been sub cooled from 212 degrees to 180 degrees. That is, it has been sub cooled by 32 degrees. When you drink 180 degree coffee, you are drinking a sub cooled liquid!
Sensible Heat & Latent Heat:
Sensible heat is heat that can be measured by a thermometer. Anytime heat is added or removed from a substance and a temperature change occurs, a sensible heat change has taken place. Since both superheat and Sub cooling are changes in temperature, they are both sensible heat processes.
When an air conditioning system cools air sensible heat has been removed. In fact, since the air is a gas or vapor and is heated far above it’s boiling
(saturation) point, it is superheated air. Yes, you are breathing superheated air
RAC BY:- Raju Kumar Das
Asst prof
GIIT Gwl

ME-803
Refrigeration & Air Conditioning as the air is hundreds of degrees above the temperature at which the gases which make up air would condense back into liquid form.
Superheated does not necessarily mean hot. And, sub cooled does not necessarily mean cold. Superheat and Sub cooling are determined by the boiling temperature of the substance and unlike water many substances have low boiling temperatures. Recalling that latent heat is the heat which is
Added to a liquid to cause it to change from a liquid to a gas (boiling) without a change in temperature, let’s go to the next step. When a gas or vapor is above it’s boiling point it is said to be superheated. Cooling the gas removes it’s superheat. When all the superheats removed from a gas, the gas will condense back into a liquid. The heat removed from saturated gas to allow it to
Condense back into a liquid is once again latent or hidden heat and is not a sensible heat process. That is, during the process of changing from a gas to a liquid it occurs at a constant temperature therefore a thermometer will not detect any temperature change. That is latent heat. Air contains water vapor or moisture.
Humid air is not comfortable. Too much humidity (moisture) in air is uncomfortable. As air containing too much moisture passes over a properly designed, installed and operating air conditioning system, the air is cooled by the air conditioning coil (evaporator) located at the indoor blower section. If the air containing the moisture is cooled to the condensing temperature (dew point) of the moisture in the air, some of the moisture will condense and deposit on the coil and fins of the cooling coil. Since the water vapor ichanging from a gas or vapor to a liquid, this is a latent heat process. The condensed water should run off the coil and be drained away. A properly operating air conditioning system both cools (a sensible heat process) and dehumidifies (a latent heat process) the air. For example, given a 3-ton residential air conditioning system, a percentage of the total capacity of the system is utilized to cool
R-503 is no longer available for general use. R-23 can be used, butR-508b, an isotropic mixture of two non-ozone-depleting refrigerants, R-23 and R-116, is superior. It is nonflammable and has zero ozone depletion potential (ODP). Table 2lists some properties of R-
508b.
RAC BY:- Raju Kumar Das
Asst prof
GIIT Gwl

ME-803
Refrigeration & Air Conditioning
The compressor for this duty may also be a standard refrigeration compressor, provided that the com-presser suction gas is superheated to at least − 43°C to avoid low-temperature metal brittleness. Operation is typically well within standard pressure, oil temperature, and discharge temperature lim-its. The refrigerant is usually R-23 or R-508b. Because the temper-natures in the low side are below
− 45°C, all piping, valves, and vessels must be of materials incompliance with the
ASME codes pertaining to these temperatures. It may be difficult to obtain compressor rating data for low-temperature applications with these refrigerants because few actual test data are available, and the manufacturer may be reluctant to be specific. Therefore, the low side should not be designed too close to the required specification. A good practice is to calico-late the actual volumetric flow rate to be handled by the compress-soar (at the expected superheat) to be certain that it can perform as required
RAC BY:- Raju Kumar Das
Asst prof
GIIT Gwl

ME-803
Refrigeration & Air Conditioning production of dry ice
1 Introduction Dry ice is CO2 in its solid form, produced by expanding liquid
CO2to atmospheric pressure. The product can be supplied as either blocks, slices or pellets and is generally packed into plastic, paper or composite bags that are stored and transported in insulated containers. Some products (particularly pellets) can also be supplied "loose" in containers, with no wrapping. Dry ice is used in practically all types of industries, mainly because of its cooling properties. It is particularly interesting for applications where "spot cooling" is needed. The most common uses are:
•Cooling c ooling of catering trolleys in airplanes, trains .. c ooling of food, pharmaceutical products, etc. during transport without direct contact between product and dry ic e (to maintain the “cold chain”) c ooling of food with direct contact between dry ice and the product (meat, grapes,...) d irect application in food mixing processes in order to maintain the temperature
Liquid state
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GIIT Gwl

ME-803
Refrigeration & Air Conditioning
CO2can exist as liquid below the criti cal temperature of 31°C and above the triple point with a temperature of -
56,6°C and 4,18 bar gauge (barg). CO2 is transported, stored and handled in liquid form, either at ambient temperature (in cylinders oruninsulated storage tanks at a pressure of 45 -
65 brag) or refrigerated (in insulated tankers and storage tanks) at a temperature range of -35 to 15°C and a pressure range of 12 to 25 barg.
The CO2in this state is liquid at its boiling point. Below the triple point 4,18 brag and 56,6°C CO2can only exist in the solid and the gas phase.
Therefore liquid CO2cannot exist at atmospheric pressure. When the liquid
CO2is depressurized below the triple point pressure of 4,18 brag to at morphemic pressure it is transformed to dry ice and gas, consequently when the liquid CO2is released to the atmosphere dense white fog of powdery solid carbon dioxide particles and vapor is produced.
Solid state (Dry ice)
The expansion of liquid CO
2 to atmospheric pressure is used to produce CO2 snow at a temperature of 78,5°C. The snow is compressed to form dry ice blocks, slices or pellets.
RAC BY:- Raju Kumar Das
Asst prof
GIIT Gwl

ME-803
Refrigeration & Air Conditioning
Claude System
–In the year 1920,
Claude developed an air liquefaction system and established lair Liquid
1) Liquid yield
2) Work requirement
3) Parametric study
In order to achieve a better performance and to approach ideality, the expansion process should be a reversible process.
•In the earlier lecture, we have seen that a J –T expansion is an irreversible isenthalpic expansion and expansion using an expansion engine is an reversible isentropic process.
•For any gas, an isentropic expansion results in lower temperature irrespective of its inversion temperature (TINV).
RAC BY:- Raju Kumar Das
Asst prof
GIIT Gwl

ME-803
Refrigeration & Air Conditioning
The schematic of the Claude System is as shown.
•It consists of a compressor, three 2 –fluid heat exchangers, a J –T expansion device and a make up gas connection.
•The system also has an expansion engine operating across the second heat exchanger as shown in the figure
RAC BY:- Raju Kumar Das
Asst prof
GIIT Gwl

ME-803
Refrigeration & Air Conditioning
RAC BY:- Raju Kumar Das
Asst prof
GIIT Gwl