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Fundementals of Refrigeration - Fluids And Pressure | ||||||||||
Fluid is "any substance that can flow, liquid or gas." Refrigerant may be classified as fluid, since, within the refrigeration cycle, it exists both as a liquid and as a vapor or gas. FLUID PRESSURE The weight of a wood or any other solid material acts as a force downward on whatever is supporting it. The force of this solid object is the overall weight of the object, and the total weight is distributed over the area upon which it lies. The weight of a given volume of water, however, acts not as a force downward on the bottom of the container holding it, but also as a force laterally on the sides of the container. If a hole is made in the side of the container below the water level, the water above the hole will be forced out because of its acting downward and sideways. Fluid pressure is the force per unit area that is exerted by gas or a liquid. It is usually expressed in terms of Psi (pound per square inch). It varies directly with the density and the depth of the liquid, and, at the same depth below the surface, the pressure is equal in all directions. Notice the difference between the terms used: force and pressure. Force means the total weight of the substance; pressure means the unit force or pressure per square inch.(3-1) Pressure = Force/Area or P = F/A | ||||||||||
HEAD
Pressure and depth have a close relationship when a fluid is involved. In hydraulics, the depth of a body of water is called the head of water. Water pressure varies directly with its depth. If there is a decrease or increase in the head of a body of water, there will be a corresponding decrease or increase in the pressure involved, as well as the weight of the water, providing the other dimensions stay the same. This relationship can be expressed in the equation:
(3-2) p = 0.433 x h
Where p = pressure in psi; h = head in feet of water.
With this relationship between pressure and depth established, we can transpose the equation so that the depth of water in a tank can be found if we know the pressure reading at the bottom of the tank. If p = 0.433 x h, then h = p/0.433 is true, by transposition.
PASCAL'S PRINCIPLE
Pascal's principle, namely, that pressures applied to a confined liquid are transmitted equally throughout the liquid, irrespective of the area over which the pressure is applied. The application of this principle enabled Pascal to invent the hydraulic press, which is capable of large multiplication of force. Fig. 3-3, illustrating this principle, shows a vessel containing fluid such as oil; the vessel has a small and large cylinder connected by pipe or tubing, with a tight-fitting piston in each cylinder. If the cross-section of the small piston is 1 square inch and the area of large piston is 30 square inches, a force of 1 lb when applied to the smaller piston will support a weight of 30 lb on the large piston, because a pressure of 1 psi throughout the fluid will be exerted.
DENSITY
From a scientific or physics viewpoint, density is the weight per unit volume of a substance, and it may be expressed in any convenient combination of units of weight and volume used, such as pounds per cubic inch or pounds per cubic foot. An equation can be formulated which expresses this relationship:
(3-4) D = W/V
Where: D = density; W = weight; V = volume;
The weight, or density of water is approximately 62.4 lb per cu ft, and it can be expressed as 0.0361 lb per cu in. (1 cu ft contains 1,728 cu in., and 62.4/1,728 = 0.0361). The densities of some other common substances are listed in Fig.3-5.
| Substance | Density, lb/cu ft | Specific Gravity |
| Water (pure) | 62.4 | 1.0 |
| Ammonia (liq. 60 deg. F) | 38.5 | 0.62 |
| Aluminum | 168 | 2.7 |
| Brass | 530 | 8.5 |
| Brick (common) | 112 | 1.8 |
| Copper | 560 | 8.98 |
| Cork | 15 | 0.24 |
| Gasoline | 41.2 | 0.66 |
| Glass | 175 | 2.8 |
| Iron (cast) | 448 | 7.2 |
| Lead | 705 | 11.3 |
| Mercury | 848 | 13.6 |
| Oil (fuel) | 448.6 | 0.78 |
| Steel | 486 | 7.8 |
| Oak | 50 | 0.8 |
| Pine | 34.2 | 0.55 |
Fig. 3-5: Density and specific gravity of some common substances.
The specific gravity of any substance is the ratio of weight of a given volume of the substance to the weight of the same volume of a given substance. (Where solids or liquids are concerned water is used as a basis for specific gravity calculations, and air or hydrogen is used as a standard for gases).
Density (solid or liquid) = specific gravity x density of water (in lb/cubic foot).
Pressure within a fluid is directly proportional to the density of the fluid. This relationship can be expressed as: p = h x D
Where: p = pressure in lb per square foot; h = head or depth below the surface in feet; D = density in pounds per cubic foot.
SPECIFIC VOLUME
The specific volume of a substance is usually expressed as the number of cubic feet occupied by 1 lb of the substance. In case of liquids, it will vary with temperature and pressure. The volume of a liquid will be affected by a change in its temperature; but, since it is practically impossible to compress liquids, the volume is not affected by change in pressure.
The volume of gas or vapor is definitely affected by any change in either its temperature or the pressure to which it is subjected. In refrigeration, the volume of vapor under the varying conditions involved is most important in selection of the proper refrigerant lines and refrigerant holding vessels.
ATMOSPHERIC PRESSURE
The earth is surrounded by a blanket of air called the atmosphere, which extends 80 or more kilometers from the surface of the earth. Air has weight and also exerts pressure known as atmospheric pressure. It has been computed that a column of air, with a cross-sectional area of one square inch and extending from the earth's surface at sea level to the limits of the atmosphere, would weigh approximately 14.7 lb. force means also the weight of a substance, and pressure means unit force per square inch; therefore standard atmospheric pressure is considered to be 14.7 psi. at sea level.
This pressure is not constant; it will vary with altitude or elevation above sea level, and there will be variations due to changes in temperature as well as water vapor content of the air.
PRESSURE OF GAS
The volume of gas is affected by a change in either the pressure or temperature, or both. There are laws that govern the mathematical calculation in computing these variables.
Boyle's Law states that volume of a gas varies inversely to its pressure if the temperature of the gas remains constant. This means that the product of the pressure times the volume remains constant, or that if the pressure of the gas doubles the new volume will be one-half of the original volume. Or it may be considered that, if the volume is doubled, the absolute pressure will be reduced by one-half.
This concept may be expressed as: p1V1 = p2V2
Where: p1 = original pressure; V1 = original volume; p2 = new pressure; V2 = new volume.
It must be remembered that p1 and p2 have to be expressed in the absolute pressure terms for the above equation to be used correctly.
EXPANSION OF GAS
Most gases will expand in volume at practically the same rate with an increase in temperature, providing that the pressure does not change. And, if the gas is confined so that its volume will remain the same, the pressure in the container will increase at about the same rate as an increase in temperature.
Theoretically, if the pressure remains constant, a gas vapor will expand or contract at the rate of 1/492 for each degree of temperature change. The result of this theory would be a zero volume at a temperature of -460 deg. F, or at 0 deg. Absolute.
Charles' Law states that the volume of gas is in direct proportion to its absolute temperature, providing that the pressure is kept constant; and the absolute pressure of gas is in direct proportion to its absolute temperature, providing the volume is kept constant. That is:
(3-6) V1/V2 = T1/T2
And
(3-7) P1/P2 = T1/T2
Where: T = absolute temperature; P = absolute pressure
Or these may be expressed also as: V1T2 = V2T1 and P1T2 = P2T1
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