During the normal cooling cycle controlled by a thermostat, as room temperature rises above the high setting on the thermostat there is a need for refrigeration. Liquid solenoid (valve A) and the built-in pilot (valve D) open, allowing refrigerant to flow. The opening of the built in pilot allows the pressure to bypass the sensing chamber of valve D. This forces it to remain wide open with resultant minimum pressure drop through the valve.
When the room temperature drops to the low setting on the thermostat, there is no longer need for referation. Solenoid valve A and pilot valve D close. They remain closed until refrigeration is again required. Hot-gas valve C and defrost water solenoid valve E remain closed during the cooling cycle.
The pilot solenoid (valve B) is a 1/8 in. ported solenoid valve that is direct operated and suitable as a liquid, suction, hot gas, or pilot valve at pressures to 300 lb.
Solenoid valve A is a one-piston, pilot-operated valve suitable for suction, liquid, or gas lines at pressures of 300 lb. It is available with a 9/16 in. or 3/4-in. port.
Solenoid valve C is a rugged, pilot-operated, two-piston valve with spring return for positive closing under the most adverse conditions. It is used for compressor unloading, and for liquid and hot-gas applications. The dual-pressure regulator (valve D) is designed to operate at two predetermined pressures without resetting or adjustment. By merely opening and closing a pilot solenoid, it is capable of maintaining either the low- or high-pressure setting.
Figure 10-8 shows a pilot light assembly. It is placed on valves when it is essential to know their condition for troubleshooting procedures.
A low-temperature defrost system with water being used to defrost the drain pan is shown in Fig. 10-9.
Figure 10-7 shows a high temperature system [above 32°F (0°C)] with no drip-pan defrost. During the normal cooling cycle, controlled by a thermostat, the room temperature may rise above the high setting of the thermostat. This indicates a need for refrigeration. The liquid solenoid (valve A), pilot solenoid (valve B), and the dual-pressure regulator (valve D) open, allowing refrigerant to flow. When solenoid (valve D) is energized. The low-pressure adjusting bonnet controls the regulator. The regulator maintains the predetermined suction pressure in the evaporator.
When the room temperature reaches the low setting on the thermostat, there is no longer need for refrigeration. At this time, solenoid valve A and solenoid valve D close and remain closed until further refrigeration is required.
The hot-gas solenoid (valve C) remains closed during the normal refrigeration cycle. When the three-position selector switch is turned to “Defrost,” liquid solenoid valve A and valve D, with a built-in pilot solenoid, close. This allows valve D to operate as a defrost pressure regulator on the high setting. The hot gas solenoid (valve C) opens to allow hot gas to enter the evaporator. When the defrost is complete, the system is switched back to the normal cooling cycle.
The system may be made completely automatic by replacing the manual switch with an electric time clock. Table 10-1 shows the valve sizes needed for this system.
To defrost ammonia evaporators, it is sometimes necessary to check the plumbing arrangement and the valves used to accomplish the task. To enable hot-gas defrost systems to operate successfully, several factors must be considered. There must be an adequate supply of hot gas. The gas should be at a minimum of 100 psig. The defrost cycle should be accurately timed. Condensate removal or storage must be provided. An automatic suction accumulator or heat reservoir should be used to protect compressors from liquid refrigerant slugs if surge drums or other evaporators are not adequate to handle the excess gas and condensates (see Fig. 10-7).
Controls must be used to direct and regulate the pressure and flow of ammonia and hot gas during refrigeration and defrost cycles.
Evaporator coils on air-conditioning units fall into two categories:
Finned-tube coil is placed in the air stream of the unit. Refrigerant vaporizes in it. The refrigerant in the tubes and the air flowing around the fins attached to the tubes draw heat from the air. This is commonly referred to as a direct expansion cooling system (see Fig. 10-4).
Shell-and-tube chiller units are used to chill water for air-cooling purposes. Usually, the refrigerant is in tubes mounted inside a tank or shell containing the water or liquid to be cooled. The refrigerant in the tubes draws the heat through the tube wall and from the liquid as it flows around the tubes in the shell. This system can be reversed. Thus, the water would be in the tubes and the refrigerant would be in the tank. As the gas passes through the tank over the tubes, it would draw the heat from the water in the tubes (see Fig. 10-5).
Figure l0-5 shows how K-12 is used in a standard vapor-compression refrigeration cycle. System water for air-conditioning and other uses is cooled as it flows through the evaporator tubes. Heat is transferred from the water to the low-temperature, low-pressure refrigerant. The heat removed from the water causes the refrigerant to evaporate. The refrigerant vapor is drawn into the first stage of the compressor at a rate controlled by the size of the guide-vane opening. The first stage of the compressor raises the temperature and pressure of the vapor. This vapor, plus vapor from the flash economizer, flows into the second stage of the compressor. There, the saturation temperature of the refrigerant is raised above that of the condenser water.
This vapor mixture is discharged directly into the condenser. There, relatively cool condenser water removes heat from the vapor, causing it to condense again to liquid. The heated water leaves the system, returning to a cooling tower or other heat-rejection device.
A thermal economizer in the bottom section of the condenser brings warm condensed refrigerant into contact with the inlet water tubes. These are the coldest water tubes. They may hold water with a temperature as low as 55°F (13°C). This subcools the refrigerant so that when it moves on in the cycle, it has greater cooling potential. This improves cycle efficiency and reduces power per ton requirements. The liquefied refrigerant leaves the condenser through a plate-type control. It flows into the flash economizer or utility vessel. Here, the normal flashing of part of the refrigerant into vapor cools the remaining refrigerant. This flash vapor is diverted directly to the second stage of the compressor. Thus, it does not need to be pumped through the full compression cycle. The net effect of the flash economizer is energy savings and lower operating costs. A second plate-type control meters the flow of liquid refrigerant from the utility vessel back to the cooler, where the cycle begins again (see Fig. 10-6).
Figure 9-81 shows how the spiral-shaped members fit together. Abetter view is shown in Fig. 9-82. The two members fit together forming crescent-shaped gas pockets. One member remains stationary, while the second member is allowed to orbit relative to the stationary member.
This movement draws gas into the outer pocket created by the two members, sealing off the open passage. As the spiral motion continues, the gas is forced toward the center of the scroll form. As the pocket continuously becomes smaller in volume it creates increasingly higher gas pressures. At the center of the pocket, the high-pressure gas is discharged from the port of the fixed scroll member. During the cycle, several pockets of gas are compressed simultaneously. This provides a smooth, nearly continuous compression cycle.
This results in a 10 to 15 percent more efficient operation than with the piston compressors. A smooth, continuous compression process means very low flow losses. No valves are required. This eliminates all valve losses. Suction and discharge locations are separate. This substantially reduces heat transfer between suction and discharge gas. There is no reexpansion volume. This increases the compressor’s heat pump capacity in low-ambient operation. Increased heat pump capacity in low ambient temperatures reduces the need for supplemental heat when temperatures drop.
During summer, this means less cycling at moderate temperatures. It also allows better dehumidification to keep the comfort level high. When temperatures rise, the scroll compressor provides increased capacity for more cooling.
During the winter, the scroll compressor heat pumps deliver more warm air to the conditioned space than conventional models.
The scroll compressor (Fig. 9-80) is being used by the industry in response to the need to increase the efficiency of air-conditioning equipment. This is done in order to meet the U.S. Department of Energy Standards of 1992. The standards apply to all air conditioners. All equipment must have a Seasonal Energy Efficiency Ratio (SEER) of 10 or better. The higher the number, the more efficient the unit is. The scroll compressor seems to be the answer to more efficient compressor operation.
The twin screw is the most common type of screw compressor used today. It uses a double set of rotors (male and female) to compress the refrigerant gas. The male rotor usually has four lobes. The female rotor consists of six lobes. Normally, this is referred to as a 4 + 6 arrangement. However, some compressors, especially air conditioners are using other variations, such as 5 + 7.
A single screw compressor is shown in Fig. 9-77. The compression process starts with the rotors meshed at the inlet port of the compressor. The rotors turn. The lobes separate at the inlet port, increasing the volume between the lobes. This increased volume causes a reduction in pressure. Thus, drawing in the refrigerant gas. The intake cycle is completed when the lobe has turned far enough to be sealed off from the inlet port. As the lobe continues to turn, the volume trapped in the lobe between the meshing point of the rotors, the discharge housing, and the stator and rotors, is continuously decreased. When the rotor turns far enough, the lobe opens to the discharge port, allowing the gas to leave the compressor (see Fig. 9-78).
Screw compressors operate more or less like pumps, and have continuous flow refrigerant compared to reciprocals. Reciprocal have pulsations. This results in smooth compression with little vibration. Reciprocals, on the other hand, make pulsating sounds and vibrate. They can be very noisy.
Screw compressors have almost linear capacity-control mechanisms. That results in excellent part-load performance. Due to its smooth operation, low vibration screw compressors tend to have longer life than reciprocals.
Centrifugals are constant speed machines. These machines surge under certain operating conditions. This results in poor performance and high power consumption at part load. Screw compressors have proven themselves in tough refrigeration applications including on-board ships. Today, screw compressors practically dominate refrigerated ships, transporting fruits, vegetables, meats, and frozen foods across the ocean with good reliability. These compressors have replaced the traditional shipboard centrifugal.
Screw compressors were developed in Germany in the 1800s. They were patented in 1883 in Italy, but not in the United States until 1905. This type of compressor is a positive-displacement compressor. That means it uses a rotor driving another rotor (twin) or gate rotors (single) to provide the compression cycle. Both methods use injected fluids to cool the compressed gas, seal the rotor or rotors, and lubricate the bearings.