Air conditioning units can be divided into two types based on their condensation form: water-cooled and air-cooled. Depending on their usage purpose, they can be divided into single cooling and cooling/heating types. Regardless of which type is used, they are composed of the following main components.
Folding refrigeration system
The main components include compressor, condenser, evaporator, expansion valve (or capillary tube, subcooling control valve), four-way valve, compound valve, one-way valve, solenoid valve, pressure switch, melt plug, output pressure regulating valve, pressure controller, storage tank, heat exchanger, collector, filter, dryer, automatic switch, shut-off valve, injection plug and other components.
The basic refrigeration system of a chiller mainly consists of four components: compressor, evaporator, condenser, and expansion valve. In addition, in order to improve the performance of the refrigeration system and achieve better usability, there are usually many auxiliary components such as liquid pipeline solenoid valves, sight glasses, liquid pipeline drying filters, high and low pressure controllers, etc. Below is a detailed introduction to the structural components of the chiller:
1. Compressor
In the chiller system, the compressor is the power source to ensure refrigeration. By using the compressor to increase the pressure of the refrigerant in the system, the refrigerant circulates in the refrigeration system to achieve the purpose of refrigeration. Compressors are classified into three types based on their structure: open type, semi closed type, and fully closed type. At present, most industrial chillers with chilled water above 0 ℃ use fully enclosed compressors, while low-temperature chillers with chilled water below 0 ℃ use semi enclosed compressors (commonly screw and reciprocating). Open type chillers are generally used in refrigeration systems with ammonia as the refrigerant.
A fully enclosed refrigeration compressor is a unit formed by a compressor and an electric motor installed in a sealed iron shell. From the appearance, only the suction and exhaust pipe joints of the compressor and the wires of the motor are visible; The compressor casing is divided into upper and lower parts. After the compressor and motor are installed, the upper and lower iron casings are welded together by electric welding. It cannot be disassembled normally, so the machine is reliable to use. In fully enclosed refrigeration compressors, there are also piston compressors and scroll compressors.
The structure of a fully enclosed vortex refrigeration compressor mainly consists of the following components: rotary inlet and outlet valves; Pressure gauge interface; Built in overload protection; Elastic machine base; Crankcase heater; Built in lubricating oil pump.
The biggest advantage of a scroll refrigeration compressor is: 1. Simple structure: the compressor body only requires 2 components (moving plate, fixed plate) to replace 15 components in a piston compressor; 2. Efficiency: The suction gas and the transformation process gas are separated to reduce heat transfer between suction and process, which can improve the efficiency of the compressor. The vortex compression process and transformation process are both very quiet.
2. Condenser
The high-temperature and high-pressure Freon in the refrigeration system enters the condenser after coming out of the compressor, releasing a large amount of heat to the cooling medium and being cooled and liquefied. So, condensers can be divided into three main types according to their cooling forms: water-cooled, air-cooled, evaporative, and water-cooled.
2-1. Water cooled type:
In a water-cooled condenser, the heat released by the refrigerant is carried away by the cooling water. Cooling water can flow through once or be recycled. When using circulating water, a cooling tower or cold water tank is required. Water cooled condensers come in various structural forms such as shell and tube, sleeve, and immersion.
The commonly used condenser for water-cooled chillers is the shell and tube condenser, which is made of steel pipes with a thickness of 5mm or more, treated with rust prevention, and has a pressure resistance of 20Kg/square centimeter. The heat exchange tube adopts high-efficiency seamless copper tube with a pressure resistance of 10 Kg/square centimeter. The end caps can be interchanged to change the direction of the water pipe. The water pipe process is multi loop, and the capacity of each condenser and compressor is coordinated. Its total thermal conductivity should be able to tolerate a scaling factor of 0.00005 Kcal/h. square centimeter ℃ -1, and the water pressure drop of the condenser should not exceed 6.5 mAq. The straight through water pipe is easy to clean and maintain.
2-2. Air cooled type:
In air-cooled condensers, the heat released by the refrigerant is carried away by the air. Its structural form is mainly composed of several sets of copper tubes. Due to the poor heat transfer performance of air, ribs are usually added outside the copper tubes to increase the heat transfer area on the air side. At the same time, ventilation fans are used to accelerate air flow and force air convection to increase heat dissipation.
2-3. Evaporative and water spray type: In this type of condenser, the refrigerant condenses inside the tube and is cooled by both water and air outside the tube.
3. Evaporator
When the liquid Freon in the refrigeration system enters the expansion valve and is throttled before being sent to the evaporator, it belongs to the vaporization process. At this time, a large amount of heat needs to be absorbed to gradually reduce the temperature of the cooled valence substance, in order to achieve the effect of refrigeration and cooling. So, according to the type of medium being cooled, it can be divided into two categories: evaporators for cooling liquids (water) (dry evaporators) and evaporators for cooling air (surface cooling evaporators).
The main focus here is on the evaporator used in the refrigeration system of chillers, which is generally a dry shell and tube evaporator. As shown in the figure, the refrigerant evaporates inside the heat exchange tube, and water flows on the shell tube side. To increase heat exchange efficiency, a 2mm thick water baffle is installed on the shell tube side to allow water to flow back and forth, achieving the purpose of producing ice brine. The container shell is made of steel pipes with a thickness of 6mm or more, which can withstand a pressure of 10 Kg/square centimeter, and is insulated with PE foam board on the outside. The heat exchange tube adopts high-efficiency seamless copper tube, which is processed into internally threaded ribbed tube through embossing technology, increasing its heat transfer area and improving heat transfer efficiency, with a pressure resistance of 20 Kg/square centimeter; The production structure of the end plate is solid, and the heat exchange tubes are combined with the end plate in an expansion type. A partition is added inside the end cover to divide the refrigerant into multiple flows to maintain the reflux of the refrigeration oil. The refrigerant liquid pipe and low-pressure pipe are connected by working valves, and their total thermal conductivity should be able to tolerate 0.086 M2 ℃/KW scaling factor. The water pressure drop through the evaporator should not exceed 6.5mAq.
4. Thermal expansion valve
In the refrigerant system flowchart of the chiller, we found a small component called the thermal expansion valve between the condenser outlet and the evaporator inlet. It is a throttling and pressure reducing component that reduces the condensation pressure of the refrigerant to the evaporation pressure. Therefore, its role in the refrigeration system is essential. It is one of the four major components of the refrigeration system, along with the refrigeration compressor, evaporator, and condenser.
4-1. Structure of Thermal Expansion Valve
The top of the expansion valve is composed of a sealed box cover corrugated film temperature sensing bag and a capillary tube, forming a closed container. Freon is filled inside to form an induction mechanism. The refrigerant filled in the induction mechanism can be the same as or different from that of the refrigeration system. For example, the refrigeration system uses F-22, and the temperature sensing bag can be filled with F-12 or F-22. The temperature sensing bag is used to sense the temperature of the superheated steam at the outlet of the evaporator. The capillary tube serves as the connecting pipe between the sealed box and the temperature sensing bag, transmitting pressure to the diaphragm. The diaphragm is formed by stamping a thin alloy sheet of about 0.2mm with a wavy cross-section. After being subjected to force, the elastic deformation performance is very good. The adjusting rod is used to adjust the opening superheat of the expansion valve. During the debugging process, it is used to adjust the elasticity of the spring. When the adjusting rod rotates inward, the spring is compressed, and when the adjusting rod rotates outward, the spring is relaxed. The transmission rod is pressed between the valve needle seat and the transmission plate to transmit pressure. The valve needle seat is equipped with a valve needle, which is used to open or close the valve hole.
4-2. Working principle of thermal expansion valve
The expansion valve senses the change in superheat at the outlet end of the evaporator through the temperature sensing bulb, causing a pressure change in the filling material in the temperature sensing system (which is a closed system composed of interconnected parts such as the temperature sensing bulb, capillary tube, transmission diaphragm, and transmission bellows) and acting on the transmission diaphragm, causing the diaphragm to move up and down. The force is then transmitted to the transmission rod through the transmission plate to push the valve needle up and down, causing the valve to close or open larger, playing a role in reducing pressure and throttling, automatically adjusting the refrigerant supply of the evaporator, and maintaining a certain degree of superheat at the outlet end of the evaporator, ensuring the full utilization of the heat transfer area of the evaporator, and reducing the occurrence of liquid impact and cylinder flushing.
4-3. Types of expansion valves (internal balance, external balance)
The pressure acting on the lower part of the transmission diaphragm in the thermal expansion valve body is the evaporation pressure after throttling (this pressure enters the space under the diaphragm through the gap between the transmission rod and the transmission plate). This structure is called an internal balance expansion valve. The pressure acting on the lower part of the transmission diaphragm in the thermal expansion valve body is not the evaporation pressure after throttling, but the pressure at the outlet end of the evaporator is introduced into the lower space structure of the transmission diaphragm through an external balance pipe, which is called an external balance type thermal expansion valve. Compared with the internal balance expansion valve, the superheat of the external balance thermal expansion valve is much smaller. Therefore, when using the external balance thermal expansion valve, it can fully utilize the heat transfer area of the evaporator and improve the effectiveness of the refrigeration device. In the case of low resistance and pressure loss in the evaporator, the internal balance thermal expansion valve can be selected; When the evaporation resistance is high, the pressure loss is significant, or there is a liquid distributor, an external balance type thermal expansion valve should be selected. For distributors, external balance expansion valves are generally used. External balance type thermal expansion valve is usually used in the chiller of refrigeration equipment.
5. Other accessories
5-1. Liquid pipeline drying filter
Usually, liquid pipeline drying filters are non removable. The internal structure adopts molecular sieve, which can remove a small amount of impurities and water in the pipeline, achieving the purpose of purifying the system. Due to the occurrence of oxides during pipeline welding and varying purity of Freon refrigerants, the Freon refrigerants we use are required to be imported. When the liquid pipeline drying filter is clogged, it will cause a decrease in suction pressure and a temperature difference at both ends of the filter. If this situation occurs, the filter needs to be replaced.
5-2. High and low pressure controller
High and low pressure controllers are protective devices in refrigeration systems. High pressure protection is an upper limit protection. When the high pressure reaches the set value, the high pressure controller disconnects, causing the compressor contactor coil to release and the compressor to stop working, avoiding damage to parts during operation under ultra-high pressure. High voltage protection is manually reset. When the compressor needs to be restarted, the reset button needs to be pressed first. Of course, before restarting the compressor, the cause of the high pressure should be checked and eliminated before the machine can operate normally. Low pressure protection is a protective device set up to prevent the refrigeration system from operating at too low a pressure. Its settings are divided into high limit and low limit. Its control principle is: the low voltage disconnection value is the pressure difference between the upper and lower limits, and the restart value is the upper limit value. The low-pressure controller is automatically reset, so operators are required to frequently observe the operation of the machine and promptly handle any alarms to avoid prolonged and frequent start stop of the compressor, which may affect its lifespan.
