Principles Of Refrigeration Pdf

[Vita Wanberg]

A chiller is simply a device that is used to remove heat from something. For industrial purposes, chillers can be thought of as a component within a complex mechanical system that is used to remove heat from a process or substance. To really understand what a chiller is, a fundamental knowledge of the principles of basic refrigeration is required.

A chiller is simply a device that used to remove heat from something. For industrial purposes, chillers can be thought of as a component within a complex mechanical system that is used to remove heat from a process or substance. To really understand what a chiller is, a fundamental knowledge of the principles of basic refrigeration is required. Welcome to Berg's School of Cool.

A). Heat is a form of energy transferred by virtue of a difference in temperature. Heat exists everywhere to a greater or lesser degree. As a form of energy it can be neither created or destroyed, although other forms of energy may be converted into heat, and vice versa. It is important to remember that heat energy travels in only one direction; from a warmer to a cooler object, substance, or area.

B). Cold is a relative term referring to the lack of heat in an object, substance, or area. Another definition describes it as the absence of heat, no process yet has been devised of achieving "absolute zero," the state in which all heat has been removed from any object, substance, or area. Theoretically this zero point would be 459.69 degrees below zero on the Fahrenheit thermometer scale, or 273.16 degrees below zero on the Celsius thermometer scale.

C). Refrigeration, or cooling process, is the removal of unwanted heat from a selected object, substance, or space and its transfer to another object, substance, or space. Removal of heat lowers the temperature and may be accomplished by use of ice, snow, chilled water or mechanical refrigeration.

E). Refrigerants, are chemical compounds that are alternately compressed and condensed into a liquid and then permitted to expand into a vapor or gas as they are pumped through the mechanical refrigeration system to cycle.

The refrigeration cycle is based on the long known physical principle that a liquid expanding into a gas extracts heat from the surrounding substance or area. (You can test this principle by simply wetting your finger and holding it up. It immediately begins to feel cooler than the others, particularly if exposed to some air movement. That's because the liquid in which you dipped it is evaporating, and as it does, it extracts heat from the skin of the finger and air around it).

The job of the refrigeration cycle is to remove unwanted heat from one place and discharge it into another. To accomplish this, the refrigerant is pumped through a closed refrigeration system. If the system was not closed, it would be using up the refrigerant by dissipating it into the surrounding media; because it is closed, the same refrigerant is used over and over again, as it passes through the cycle removing some heat and discharging it. The closed cycle serves other purposes as well; it keeps the refrigerant from becoming contaminated and controls its flow, for it is a liquid in some parts of the cycle and a gas or vapor in other phases.

Let's look at what happens in a simple refrigeration cycle, and to the major components involved. Two different pressures exist in the cycle - the evaporating or low pressure in the "low side," and the condensing, or high pressure, in the "high side." These pressure areas are separated by two dividing points: one is the metering device where the refrigerant flow is controlled, and the other is at the compressor, where vapor is compressed.

The metering device is a point where we will start the trip through the cycle. This may be a thermal expansion valve, a capillary tube, or any other device to control the flow of refrigerant into the evaporator, or cooling coil, as a low-pressure, low-temperature refrigerant. The expanding refrigerant evaporates (changes state) as it goes through the evaporator, where it removes the heat from the substance or space in which the evaporator is located.

Heat will travel from the warmer substance to the evaporator cooled by the evaporation of the refrigerant within the system, causing the refrigerant to "boil" and evaporate, changing it to a vapor. This is similar to the change that occurs when a pail of water is boiled on the stove and the water changes to steam, except that the refrigerant boils at a much lower temperature.

Now this low-pressure, low-temperature vapor is drawn to the compressor where it is compressed into a high-temperature, high-pressure vapor. The compressor discharges it to the condenser, so that it can give up the heat that it picked up in the evaporator. The refrigerant vapor is at a higher temperature than the air passing across the condenser (air-cooled type); or water passing through the condenser (water-cooled type); therefore that is transferred from the warmer refrigerant vapor to the cooler air or water.

The liquid refrigerant travels now to the metering device where it passes through a small opening or orifice where a drop in pressure and temperature occurs, and then it enters into the evaporator or cooling coil. As the refrigerant makes its way into the large opening of the evaporator tubing or coil, it vaporizes, ready to start another cycle through the system.

The refrigeration system requires some means of connecting the basic major components - evaporator, compressor, condenser, and metering device - just as roads connect communities. Tubing or "lines" make the system complete so that the refrigerant will not leak out into the atmosphere. The suction line connects the evaporator or cooling coil to the compressor, the hot gas or discharge line connects the compressor to the condenser, and the liquid line is the connecting tubing between the condenser and the metering device (Thermal expansion valve). Some systems will have a receiver immediately after the condenser and before the metering device, where the refrigerant is stored until it is needed for heat removal in the evaporator.

There are many different kinds and variations of the refrigeration cycle components. For example, there are at least a half dozen different types of compressor, from the reciprocating, piston through a screw, scroll and centrifugal impeller design, but the function is the same in all cases - that of compressing the heat laden vapor into a high-temperature vapor.

The same can be said of the condenser and evaporator surfaces. They can be bare pipes, or they can be finned condensers and evaporators with electrically driven fans to pass the air through tem, or with a condenser pump to pump the water through a water-cooled condenser.

The mechanical refrigeration system described above is essentially the same whether the system be a domestic refrigerator, a low-temperature freezer, comfort air conditioning system, industrial chiller, or commercial cooling equipment. Refrigerants will be different and size of the equipment will vary greatly, but the principle of operation and the refrigeration cycle remains the same. Thus, once you understand the simple actions that are taking place within the refrigeration mechanical cycle you should have a good understanding how a refrigeration system works.

Disclaimer - While Berg Chilling Systems Inc. ("Berg") makes reasonable efforts in providing accurate information, we make no representations or warranties regarding the accuracy of any content therein. We assume no liability or responsibility for any typographical, content or other errors or omissions. We reserve the right to modify the content of this documentation without advance notice.

You will see that different substances vary in their capacity to absorb or give up heat. The specific heat values of most substances will vary with a change in temperature; some vary only a slight amount, while others can change considerably.

Suppose that two containers are placed on a heating element or burner side by side, one containing water and the other an equal amount, by weight, of olive oil. You would soon find that the temperature of the olive oil increases at a more rapid rate than that of the water, demonstrating that olive oil absorbs heat more rapidly than water.

If the rate of the temperature increase of the olive oil was approximately twice that of the water, it could be said that olive oil requires only half as much heat as water to increase its temperature one degree Fahrenheit. Based on the value 1.0 for the specific heat of water, it would show that the specific heat of olive oil must be approximately 0.5, or half that of water. (The table of specific heat of substances shows that olive oil has a value of 0.47).

The specific heat of a substance also will change, with a change in the state of substance. Water is a very good example of this variation in specific heat. Specific heat of water is 1.0; but as solid ice, its specific heat approximates 0.50; and a similar value is applied to steam 0.48; the gaseous state of the water.

Heat that can be felt or measured is called sensible heat. It is the heat that causes a change in temperature of a substance, but not a change in state. Substances, whether in a solid, liquid, or gaseous state, contain sensible heat to some degree, as long as their temperatures are above absolute zero. Equations used for solution of heat quantity, and those used in conjunction with specific heats, might be classified as being sensible heat equations, since none of them involve any change of state.

Under a change of state, most substances will have a melting point at which they will change from solid to a liquid without any increase in temperature. At this point, if the substance is in a liquid state and heat is removed from it, the substance will solidify without a change in its temperature. The heat involved in either of these processes (changing from a solid to a liquid or from liquid to a solid), without a change in temperature, is known as the latent heat of fusion.

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