Pressure
Pressure is a force that acts on an area. Stated in a formula, it becomes:
P = F / A
where F = force
A = area
P = pressure
The unit of measurement of pressure in the English system is the pound per square foot or pounds per square inch (psi). The metric unit of pressure is the kilopascal (kPa). Pressure measuring elements trans- late changes or differences in pressure into motion. The three types most commonly used are the diaphragm, the bellows, and the Bourbon spring tube.
Pressure Indicating Devices
Pressure indicating devices are most important in the refrigeration field. It is necessary to know the pressures in certain parts of a system to locate trouble spots.
The diaphragm is a flexible sheet of material held firmly around its perimeter so there can be no leakage from one side to the other. See Fig. 2-5. Force applied to one side of the diaphragm will cause it to move or flex. Diaphragms, in some cases, are a made of a flat sheet of material with a limited range of motion. Other diaphragms are at least one corrugation or fold. This allows more movement at the point where work is produced.
where F = force
A = area
P = pressure
P = F
A
Fig. 2-5 Pressure sensing element, diaphragm type.
Some types of pressure of pressure controllers require more motion per unit of force applied. To accomplish the desired result, the diaphragm is joined to the housing by a section with several convolutions or folds called bellows. Thus, the diaphragm moves in response to pressure changes.
Each holds only a small amount. See Fig. 2-6. The bellow element may be assembled to extend or to compress as pressure is applied. The bellows itself act as a spring to return the diaphragm section to the original position when the pressure differential is reduced to zero. If a higher spring return rate is required, to match or define the measured pressure range, then an appropriate spring is added.
Fig. 2-6 Pressure sensing element, bellows type. (Johnson)
One of the most widely used types of pressure measuring elements is the Bourdon spring tube, dis- cussed in Chap. 1. It is readily adaptable to many types of instruments. See Fig. 2-7. The Bourdon tube is a
Fig. 2-7 Pressure sensing element, Bourdon spring tube type. (Johnson)
flattened tube bent into a spiral or circular form closed at one end. When fluid pressure is applied within the tube, the tube tends to straighten or unwind. This pro- duces motion, which may be applied to position an indicator or actuate a controller.
Pressure of Liquids and Gases
Pascal’s law states that when a fluid is confined in a container that is completely filled, the pressure on the fluid is transmitted at equal pressure on all surfaces of the container. The pressure of a gas is the same on all areas of its container.
Atmospheric Pressure
The layer of air that surrounds the earth is several miles deep. The weight of the air above exerts pressure in all directions. This pressure is called, atmospheric pressure. Atmospheric pressure at sea level is 14.7 psi. On converting, it is 1.013 ´ 105 N/m2.
The instrument used to measure atmospheric pressure is called a barometer. Two common barometers are the aneroid barometer and the mercury barometer. The aneroid barometer has a sealed chamber containing a partial vacuum. As the atmospheric pressure increases, the chamber is compressed causing the needle to move. As the atmospheric pressure decrease, the chamber expands, causing the needle to move in the other direction. A dial on the meter is calibrated to indicate the correct pressure.
The mercury barometer has a glass tube about 34 in. long. The tube holds a column of mercury. The height of this column reflects the atmospheric pressure. Standard atmospheric pressure at sea level is indicated by 29.92 in. of mercury. That converts to 759.96 mm.
Gage Pressure
Gage pressure is the pressure above or below atmospheric pressure. This is the pressure measured with most gages. A gage that measures both pressure and vacuum is called a compound gage. Vacuum is pressure that is below atmospheric pressure. A gage indicates zero pressure before you start to measure. It does not take the pressure of the atmosphere into account. In the customary system, gage pressure is measured in pounds per square inch (psi).
Absolute Pressure
Absolute pressure is the sum of the gage pressure and atmospheric pressure. This is abbreviated as psia. A good example of this is the pressure in a car tire. This is usually 28 psi. That would be 42.7 psia. For example:
Example 2
psi (gage) = Atmospheric pressure = Absolute pressure = 28 psi
14.7 psi
- psi
Head pressure = 160 lbs Suction pressure = 10 in. of vacuum
Absolute head pressure = 160 + 15 = 175 lbs
The abbreviation for pounds per square inch gage is psig. The abbreviation for pounds per square inch absolute is psia. Absolute is found by adding 14.7 to
Absolute suction pressure =
30 – 10 = 20/2 = 10
the psig. However, the atmospheric pressure does vary with altitude. In some cases, it is necessary to convert to the atmospheric pressure at the altitude where the pressure is being measured. This small difference can make a tremendous difference in correct readings of psia. To convert psi to kPa (kilopascals), the metric unit of pressure, multiply psi by 6.9.
Compression Ratio
Compression ratio is defined as the absolute head pressure divided by the absolute suction pressure.
Compression ratio = absolute head pressure/ absolute suction pressure
Example 1
When the gage reading is 0 or above.
Absolute head pressure = gage reading
- 15 lbs (14.7 actually)
Absolute suction pressure = gage reading
- 15 lbs (14.7 actually)
Example 2
When the low side reading is in vacuum range.
Absolute head pressure = gage reading
- 15 lbs (14.7 actually) Absolute suction pressure =
30 – gage reading in inches / 2
The calculation of compression ratio can be illustrated by the following.
Example 1
Head pressure = 160 lbs
Suction pressure = 10 lbs
Compression ratio = absolute head pressure/absolute suction pressure
Compression ratio = absolute head pressure/absolute suction pressure
= 175/10 = 17.5:1
The preceding examples show the influence of back pressure on the compression ratio. A change in the head pressure does not produce such a dramatic effect. If the head pressure in both cases were 185 lb, the compression ratio in Example 1 would be 8:1, and in Example 2 it would be 20:1.
A high compression ratio will make a refrigeration system run hot. A system with a very high compression ratio may show a discharge temperature as much as 150°F [65.6°C] above normal. The rate of a chemical reaction approximately doubles with each 18°F (7.8°C) rise in temperature. Thus, a system running an abnormally high head temperature will develop more problems than, a properly adjusted system. The relationship between head pressure and back (suction) pressure, wherever possible, should be well within the accepted industry bounds of a 10:1 compression ratio.
It is interesting to compare, assuming a 175-lb heat pressure in both cases: Refrigerant 12 (R-12) versus Refrigerant 22 (R-22) operating at -35°F (-37°C) coil. At a -35°F (-37°C) coil, as described, the R-22 system would show a 10.9:1 compression ratio while the R-12 system would be at 17.4:1. The R-22 system is a borderline case. The R-12 system is not in the safe range and it would run very hot with all of the accompanying problems.
A number of other factors will produce serious high-temperature conditions. However, high compression ratio alone is enough to cause serious trouble. The thermometer shown in Fig. 2-8 reads temperature as a function of pressure. This device reads the pressure of R-22 and R-12. It also indicates the temperature in degrees Fahrenheit on the outside scale.
