Fundementals of Refrigeration - Matter and Heat Behavior

by Oldrich Bocek(1939-2003)
Thermal Management Expert
Berg Chilling Systems Inc.

A). STATES OF MATTER

All known matter exists in one of three physical forms or states: solid, liquid, or gaseous. There are distinct dissimilarities among these physical states namely:

1. Matter in solid state will retain its quantity, shape, and physical dimensions. A cubic foot of wood will retain its weight, size, and shape even if moved from place to place.

2. Matter in a liquid state will retain its quantity and size but not its shape. The liquid will always conform to the occupying container. If a cubic foot of water in a container measuring 1 foot on each side is transferred to a container of different rectangular dimensions, the quantity and volume of the water will be the same although the dimension will change.

3. Matter in gaseous state does not have a tendency to retain either its size or its shape. If a one foot cylinder containing steam or some other gas is connected to a 2-cubic foot cylinder on which a vacuum has been drawn, the vapor will expand to occupy the volume of the large cylinder. Although these specific differences exist in the three states of matter, quite frequently, under changing conditions of pressure and temperature, the same substance may exist in any one of the three states, such as a solid, a liquid, or vapor (ice, water, and steam, for example). Solids always have some definitive shape, whereas liquids and gases have no definitive shape of their own, but will conform to the shape of their containers.

B). MOLECULAR MOVEMENT

All matter is composed of small particles known as molecules, for the present we will concern ourselves only with the molecule, the smallest particle into which any matter or substance can be broken down and still retain its identity. Molecules vary in shape, size, and weight. In physics we learn that molecules have a tendency to cling together. When heat energy is applied to a substance it increases the internal energy of the molecules, which increase their motion or velocity of movement. With this increase in the movement of the molecules, there is also rise or increase in the temperature of the substance. When heat is removed from a substance, it follows that the velocity of the molecular movement will decrease and also that there will be a decrease or lowering of the internal temperature of the substance.

C). CHANGE OF STATE

When a solid substance is heated, the molecular motion is chiefly in the form of rapid motion back and forth, the molecules never moving far from their normal or original position. But at some given temperature for that particular substance, further addition of heat will not necessarily increase the molecular motion within the substance; instead, the additional heat will cause some solids to liquefy (change into a liquid). Thus the additional heat causes a change of state in the material.

The temperature at which this change of state in a substance takes place is called its melting point. Let us assume that a container of water at 70 deg F, in which a thermometer has been placed, is left in the freezer for hours. When it is taken from the freezer, it has become a block of ice - solidification has taken place. Let us further assume that the thermometer in the ice block indicates a temperature of 20 deg F.

If it is allowed to stand at room temperature, heat from the room air will be absorbed by the ice until the thermometer indicates a temperature of 32 deg F, when some of the ice will begin to change into water. With heat continuing to transfer from the room air to the ice, more ice will change back into the water; but the thermometer will continue to indicate a temperature a temperature of 32 deg F until all the ice has melted. Liquefaction has now taken place.

As mentioned, when all the ice is melted, the thermometer will indicate a temperature of 32ºF, but the temperature of the water will continue to rise until it reaches or equals room temperature. If sufficient heat is added to the container of water through outside means such as a burner, the temperature of the water will increase until it reaches 212ºF, at this temperature, and under "standard" atmospheric pressure, another change will take place - vaporization. Some of the water will vaporize into steam and, with the addition of more heat, all of the water will vaporize into steam; yet the temperature of the water will not increase above 212ºF.

Thus far we have learned bow solids can change into liquid, and how a liquid can change in to a vapor but it is possible for a substance to undergo a physical change through which solid will change directly into a gaseous state without first melting into a liquid. This is known as a sublimation. As an example, dry ice (CO2) at atmospheric conditions sublimes directly into vapor. Let us review these changes of state: a) SOLIDIFICATION - a change from a liquid to a solid. LIQUEFACTION - a change from a solid to a liquid. VAPORIZATION - a change from a liquid to a vapor. CONDENSATION - a change from a vapor to a liquid. SUBLIMATION - a change from a solid to a vapor without passing through the liquid state.

MEASUREMENTS

Most of us are acquainted with common measurement, such as those pertaining to length, weight, volume, etc.; but now we move into other types of measurement, such as those of heat intensity, heat quantity, and energy conversion units.

HEAT INTENSITY

Heat is a form of energy which is not measurable in itself; but the heat intensity, or temperature of a substance, can be measured. A unit of the intensity of heat is called the degree, measured on the temperature scale. In the discussion of state of matter, temperature was discussed, as was the addition or removal of heat. Relatively, water is colder than steam; yet it is, at the same time, warmer than ice. Temperature scales were formulated through use of glass tubes with similar interior diameter and reservoir for the liquid - such as mercury - that will expand and rise up in the tube when heated.

The Fahrenheit thermometer or scale is based on the relative position of the mercury in the thermometer when water is at the freezing point and when water is boiling. the distance between these two points was divided into 180 equal portions or parts called degrees. The point where water either will freeze, or ice will melt, under normal atmospheric conditions, was labeled as 32 degrees; whereas the location, or point on the thermometer where water will boil was labeled 212 degrees; whereas the thermometer has been the one most commonly used in most types of refrigeration engineering work. A Celsius thermometer formerly called a Centigrade thermometer, is used in chemistry and physics, especially in continental Europe, south Americas and Asia.

A frequently asked question is why the boiling point of water and the melting point of ice where used as the standard for both thermometers. These points or temperatures were chosen because water has a very constant boiling and freezing temperature, and water is a very common substance.

TEMPERATURE CONVERSION

Most frequently a conversion from one temperature scale to other is made by the use of a conversion table, but if one is not available, the conversion can be done easily by a formula using these equations:

(2-1)    Deg. F = 1.8 ºC + 32

Deg. F = 5/9 ºC + 32

(2-2)   Deg. C = (ºF - 32)/1.8

Deg. C = 5/9(ºF - 32)

Thus far, in measurement of heat intensity, we have located two definitive reference points - the freezing point and the boiling point of water on both the Fahrenheit and Celsius scales. We now must locate still a third definite point - absolute zero. This is the point where, it is believed all molecular action ceases. As already noted on the Fahrenheit temperature scale, this is about 460 Deg. below zero, -460 Deg. F, while on the Celsius scale it is about 273 Deg. below zero, or -273 Deg. C. Certain basic laws, are based on the use of absolute temperatures. If a Fahrenheit reading is given, the addition of 460 Deg. to this reading will convert it to Degrees Rankin or Deg. R; whereas if the reading is from the Celsius scale, the addition of 273 Deg. will convert it to degrees Kelvin, Deg. K.

HEAT QUANTITY

Heat quantity is different from heat intensity, because it takes into consideration not only the temperature of the fluid or substance being measured but also its weight. The unit of heat quantity is the British thermal unit (Btu). Water is used as a standard for this unit of heat quantity; a Btu is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit at sea level.

Two Btu will cause a change in temperature of two degrees Fahrenheit of one pound of water; or it will cause a change in temperature of one degree Fahrenheit of two pounds of water. Therefore, when considering a change in temperature of water, the following equation may be utilized:

(2-3)   Btu = W x TD

Where    Change in heat (in Btu) = Weight (in pounds) x Temperature Difference.

SPECIFIC HEAT

The specific heat of a substance is the quantity of heat in Btu's required to change the temperature of one pound of substance one degree Fahrenheit. Btu is the amount of heat necessary to increase the temperature of one pound water one degree Fahrenheit, or to lower the temperature of the same weight of water by the same unit of measurement on a thermometer.

Therefore, the specific heat of water is 1.0; and water is the basis for the specific heat table in figure 2-8

 Water 1 Ice 0.5 Air (dry) 0.24 Steam 0.48 Aluminum 0.55 Brass 0.09 Lead 0.03 Iron 0.1 Mercury 0.03 Copper 0.09 Alcohol 0.6 Kerosene 0.5 Olive oil 0.47 Salt Brine 20% 0.85 R-22 0.26 R-12 0.21

Fig. 2-8 Specific Heat of Common substances Btu/lb/ºF.

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).

Equation (2-3) from previous discussion can now be stated as:

(2-4)    Btu = W x c x TD

Where    c = specific heat of a substance; W = the weight of substance; and TD = temperature difference.

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.

Example: determine the amount of Btu which must be removed to cool 40 lb. of 20% salt brine from 60ºF to 20ºF.

Btu = W x c x TD

Btu = 40 lb. x 0.85 x (60ºF - 20ºF)

Btu = 1360

SENSIBLE HEAT

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.

LATENT HEAT

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.

Figure 2-9 shows the relationship between temperature in Fahrenheit degrees and both sensible and latent heat in Btu's.

Figure 2-9 Chart demonstrating sensible and latent heat relationships in melting ice, changing ice to water and water to steam.

As pointed out earlier, the specific heat of water is 1.0 and that of ice is 0.50, which is the reason for the difference in slope of the lines denoting the solid (ice) and liquid (water). To increase the temperature of the ice from -40ºF to 32ºF requires only 36 Btu of heat. (-40ºF to 32ºF = 72ºF temperature change). (Btu = 1 lb. x 0.50 x 72 = 36). From B to C, 144 Btu were added to melt the ice. The temperature did not change from B to C. From C to D, 180 Btu were added to heat the water from 32ºF to 212ºF. From D to E, 970 Btu were added to vaporize water. Note that the temperature did not change form D to E.

The derivation of the word latent is from the Latin word for hidden. This is hidden heat, which does not register on the thermometer, nor can it be felt. Needless to say, there is no increase or decrease in the molecular motion within the substance, for it would show up as a change in temperature on a thermometer.

(2-6)   Btu = (W1 x c1 x TD1)

+ (W1 x latent heat)

+ (W2 x c2 x TD2)

Another type of latent heat that must be taken into consideration when total heat calculations are necessary is called latent heat of vaporization. This is the heat that one pound of a liquid absorbs while being changed into the vapor stage. Or it can be classified as the latent heat of condensation; for, when sensible heat is removed from the vapor to the extent that it reaches the condensing point, the vapor condenses back into the liquid form.

The absorption of the amount of heat necessary for the change of state from a liquid to a vapor by evaporation, and the release of that amount of heat necessary for the change of state  from a vapor back to the liquid by condensation are the main principles of the refrigeration process, or cycle. Refrigeration is the transfer of heat by the change in state of the refrigerant.

SECOND LAW OF THERMODYNAMICS

The second law of thermodynamics, states that heat transfer in only one direction - downhill; and this takes place through one of the three basic methods of heat transfer. A. Conduction, B. Convection, C. Radiation.

CONDUCTION

Conduction is described as the transfer of heat between closely-packed molecules of the substance, or between substances that are touching or in good contact with one another. When the transfer of heat occurs in a single substance, such as a metal rod with one end in a flame, movement of heat continues until there are is a temperature balance throughout the length of the rod.

If the rod is immersed in water, the rapidly moving molecules on the surface of the rod will transmit some heat to the molecules of water, and still another transfer of heat by conduction takes place. As the outer surface of the rod cools off, there is still some heat within the rod, and this will continue to transfer to the outer surfaces of the rod and then to the water, until a temperature balance is reached.

The speed with which heat will transfer by means of conduction will vary with different substances or materials if the substances or materials are of the same dimensions. The rate of heat transfer will vary according to the ability of the materials or the substances to conduct heat. Solids, on the whole, are much better conductors than liquids; and in turn, liquids conduct heat better than gases or vapors.

Most metals, such as gold, silver, copper, steel, and iron, conduct heat fairly rapidly, whereas other solids such as glass, wood, polyurethane, or other fibrous building materials transfer heat at a much slower rate and are therefore used as insulators.

Copper is an excellent conductor of heat, as is aluminum. These substances are ordinarily used in refrigeration evaporators, condensers, and refrigeration pipes connecting the various components of a refrigerant system, although iron and carbon steel is occasionally used with some large refrigerant installations.

The rate at which the heat may be conducted through various materials is dependent on such factors as (a) thickness of material, (b) its cross-sectional area, (c) the temperature difference between the two sides of the material, (d) the heat conductivity (k factor) of the material, and (e) the time duration of the heat flow.

Figure 2-10 is a table of heat conductivity (k factors) of some common materials.

 Material Conductivity (k) Plywood 0.80 Glass Fiber-organic bonded 0.25 Expanded polystyrene insulation 0.25 Expanded polyurethane insulation 0.16 Cement Mortar 5.0 Stucco 5.0 Brick (common) 5.0 Hard woods (maple, oak) 1.10 Soft woods (fir, pine) 0.80 Gypsum Plaster (Sand aggregate) 5.6

Figure 2-10 Conductivities for common building and insulating materials.

NOTE: The k factors are given in Btu/hr/ft sq/ºF/in. of thickness of the material. These factors may be utilized through use of the following equation:

(2-7)   Btu =(A x k x TD)/X

Where:    A = Cross-sectional area in ft sq. k = Heat conductivity in Btu/hr. TD = Temperature difference between the two sides. X = Thickness of material in inches.

Metals with a high conductivity are used within the refrigeration system itself because it is desirable that rapid transfer of heat occur in both evaporator and condenser. The evaporator is where heat is removed from the conditioned space or substance; the condenser dissipates this heat to another medium or space.

In the case of the evaporator, the substance or air is at a higher temperature than the refrigerant within the tubing and there is a transfer of heat downhill; whereas in the condenser the refrigerant vapor is at a higher temperature than the cooling medium traveling through the condenser, and here again there is a downhill transfer of heat.

Plain tubing, whether copper, or aluminum, or another metal, will transfer heat according to its conductivity or k factor, but this heat transfer can be increased through addition of fins on the tubing. They will increase the area of heat transfer surface, thereby increasing the overall efficiency of the system. If additions of fins doubles the surface area, it can be shown by the use of Eq. (2-7) that the overall heat transfer should itself be doubled, when compared to that of plain tubing.

CONVECTION

Another means of heat transfer is by motion of the heated material itself and is limited to liquid or gas. When a material is heated, convection currents are set up within it, and the warmer portions of it rise, since heat brings about the decrease of a fluid's density and an increase of its specific volume.

Air within a refrigerator and water being heated in a pan are prime examples of the result of convection currents. The air in contact with the cooling coil on a refrigerator becomes cool and therefore more dense, and begins to fall to the bottom of the refrigerator. In doing so, it absorbs heat from the product and the walls of the refrigerator, which, through conduction, has picked up heat from the room.

After heat has been absorbed by the air it expands, becoming lighter, and rises until it again reaches the cooling coil where heat is removed from it. The convection cycle repeats as long as there is a temperature difference between the air and the coil. In commercial-type units, baffles may be constructed within the box in order that the convection currents will be directed or take the desired patterns of air flow around the cooling coil.

Water heated in a pan will be affected by the convection currents set up within it through application of heat. The water nearest the heat source, in absorbing heat, becomes warmer and expands. As a result it becomes lighter, it rises and is replaced by the cooler more dense water. This process will continue until all of the water is at the same temperature.

Convection currents as explained here are natural, and, as is the case of the refrigerator, a natural flow is a slow flow. In many cases, convection must be increased through use of fans or blowers and, in the case of liquids, pumps are used for forced circulation to transfer heat from one place to another.

A third means of heat transfer is through radiation by waves similar to light or sound waves. The sun's rays heat the earth by means of radiant heat waves, which travel in a straight path without heating the intervening matter of air. The heat from a light bulb or from a hot stove is radiant in nature and is felt by those near them, although the air between the source and the object, which the rays pass through, is not heated. If you have been relaxing in the shade of a building or a tree on a hot sunny day and move into direct sunlight, the direct impact of the heat waves will hit like a sledge hammer even though the air temperature in the shade is approximately the same as in the sunlight.

Al low temperatures there is only a small amount of radiation, and only minor temperature differences are noticed; therefore radiation has very little effect in actual process of refrigeration itself. But results of radiation from direct solar rays can cause an increased refrigeration load in a building air conditioning system. Radiant heat is readily absorbed by dark or dull materials or substances, whereas light-colored surfaces or materials will reflect radiant heat waves, just as they do light rays.

When radiant heat or energy (since all heat is energy) is absorbed by a material or substance it is converted into sensible heat - that which can be felt or measured. Every body or substance absorbs radiant energy to some extent, depending upon the temperature difference between the specific body or substance and other body or substances. Every substance will radiate energy as long as its temperature is above absolute zero and another substance within its proximity is at a lower temperature.

INSULATION

Any material that deters or helps to prevent the transfer of heat by any means is called and may be used as insulation. Of course, no material will stop completely the flow of heat. If there were such a substance, it would be very easy to cool a given space down to desired temperature and keep it there.

such substances as cord, glass fibers, mineral wool, polyurethane and polystyrene foams are good examples of insulating materials; but numerous other substances are used in insulating refrigerated spaces or buildings.

Insulation should be fire and moisture resistant, and also vermin proof. Low temperature components and boxes require an insulation that is vapor-resistant, such as unicellular foam, so that water vapor will not readily penetrate through into the insulation and condense there, reducing the insulating efficiency.

REFRIGERATION EFFECT - "TON"

A common term that has been used in refrigeration work to define and measure capacity or refrigeration effect is called a ton of refrigeration. It is the amount of heat absorbed in melting a tone of ice (2,000 lb) over a 24-hour period.

The ton of refrigeration is equal to 288,000 Btu. This may be calculated by multiplying the weight of ice (2,000 lb) by the latent heat of fusion (melting) of ice (144 Btu/lb). Thus

2.000 lb x 144 Btu/lb = 288,000 Btu

in 24 hours or 12,000 Btu per hour (288,000 / 24). Therefore, one ton of refrigeration = 12,000 Btu/hr.

SUMMARY

The change of state of matter can be effected by adding or taking away heat. Heat effect or intensity can be measured by the use of thermometers. Heat always travels from a warmer condition to a cooler condition. Substances have different capacities to absorb heat. Heat exists in two forms: sensible and latent. The unit of measure to express heat quantity is the Btu. Heat can be transferred by several methods: conduction, convection and radiation. An insulator is a substance that will retard the flow of heat.

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