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Selection points and troubleshooting of screw chiller


Product selection points

1. The main control parameters of screw chiller are refrigeration performance coefficient, rated cooling capacity, input power and refrigerant type.
2. The selection of chiller should be considered according to the cooling load and application. For a refrigeration system with low load and long operating time, a multi-head piston compressor or screw compressor should be selected to facilitate adjustment and energy saving.
3. When selecting a chiller, priority should be given to the unit with a higher coefficient of performance. According to statistics, the operating time of a general chiller under 100% load throughout the year accounts for less than 1/4 of the total operating time. The total operating time for the 100%, 75%, 50%, and 25% loads is approximately 2.3%, 41.5%, 46.1%, 10.1%. Therefore, when selecting a chiller, priority should be given to models with relatively flat efficiency curves. At the same time, the adjustment range of the chiller load should be considered in the design and selection. The multi-head screw chiller has excellent partial load performance and can be selected according to actual conditions.
4. When choosing a chiller, pay attention to the conditions of nominal working conditions. The actual cooling capacity of the chiller is related to the following factors:
a) cold water outlet temperature and flow;
b) Cooling water inlet temperature, flow rate and fouling factor.
5. When selecting a chiller, pay attention to the normal working range of this type of unit, mainly because the current limit of the main motor is the current value of the shaft power under nominal conditions.
6. In the design and selection, it should be noted that at the nominal operating flow rate, the outlet temperature of the cold water should not exceed 15 ℃, and the outdoor dry bulb temperature of the air-cooled unit should not exceed 43 ℃. Whether the use range is allowed, and the power of the main motor is sufficient.
Common faults and solutions

1. High voltage fault

The compressor discharge pressure is too high, causing the high-pressure protection relay to operate. The compressor discharge pressure reflects the condensing pressure, the normal value should be 1.4 ~ 1.6MPa, and the protection value is set to 2.0MPa. If the pressure is too high for a long period of time, it will cause the compressor to run too much current, burn the motor easily, and damage the valve flap of the compressor exhaust port. The causes of high voltage faults are as follows:
(1) The cooling water temperature is too high, and the condensation effect is poor. The rated working condition of the cooling water required by the chiller is 30 ~ 35 ° C. High water temperature and poor heat dissipation will inevitably lead to high condensation pressure. This phenomenon often occurs in high temperature seasons. The reason for the high water temperature may be: cooling tower failure, such as the fan is not turned on or even reversed, the water distributor does not turn on, the cooling water temperature is high and rising rapidly; the outside temperature is high, the water path is short, and the amount of recyclable water In this case, the temperature of the cooling water is generally maintained at a relatively high level, which can be solved by increasing the storage tank.
(2) The cooling water flow is insufficient to reach the rated water flow. The main performance is that the pressure difference between the inlet and outlet of the unit becomes smaller (compared to the pressure difference at the beginning of the system operation), and the temperature difference becomes larger. The cause of insufficient water flow is the lack of water or air in the system. The solution is to install an exhaust valve at a high place in the pipeline for exhausting; the pipe filter is blocked or selected too thin, and the water permeability is limited. You should choose a suitable filter and Clean the filter regularly; the pump selection is small and not compatible with the system.
(3) The condenser is fouled or blocked. The condensed water is generally tap water. It is easy to scale at temperatures above 30 ° C. Because the cooling tower is open and directly exposed to the air, dust and foreign matter can easily enter the cooling water system, causing the condenser to be dirty and the heat exchange area small , Low efficiency, but also affect water flow. The performance is that the pressure difference and temperature difference between the inlet and outlet water of the unit become larger, the temperature above and below the condenser is very high, and the copper tube from the condenser is hot. The unit should be backwashed regularly and chemically cleaned and descaled if necessary.
(4) Too much refrigerant charge. This situation generally occurs after maintenance, and manifests itself as high suction and discharge pressures and equilibrium pressures, and high compressor operating currents. Bleed according to the suction and discharge pressure and equilibrium pressure and running current under normal operating conditions until normal.
(5) Non-condensable gases such as air and nitrogen are mixed in the refrigerant. This situation usually occurs after maintenance, and the vacuum is not complete. Can only be drained, re-evacuated and refilled with refrigerant.
(6) False alarms caused by electrical faults. Because the high-voltage protection relay is wet, bad contact, or damaged, the unit electronic board is wet or damaged, and a communication failure causes a false alarm. This kind of false failure often means that the HP fault indicator on the electronic board is off or slightly off, manual reset of the high-voltage protection relay is invalid, the computer displays "HP RESET", or disappears automatically, and the measured compressor running current is normal, and the suction and exhaust pressure is also normal.

2. Low voltage fault

The compressor suction pressure is too low, causing the low-pressure protection relay to operate. 2MPa。 The compressor suction pressure reflects the evaporation pressure, the normal value should be 0.4 ~ 0. 6MPa, the protection value is set to 0. 2MPa. If the suction pressure is low, the return air volume will be small and the cooling capacity will be insufficient, resulting in a waste of electrical energy. Poor heat dissipation for the return air cooled compressor motor will easily damage the motor. The causes of low-voltage faults are as follows:
(1) Insufficient or leaking refrigerant. If the refrigerant is insufficient and only partly leaks, the equilibrium pressure may be higher when the machine is stopped, and the suction pressure is lower and the exhaust pressure is lower after the machine is turned on. The compressor running current is smaller and the operating time is shorter. The computer displays "LP CURRENT", and at the same time the unit electronic board LP fault indicator lights up. After a few seconds, the computer displays "LP RESET", and the unit electronic board LP fault indicator turns off. If most of the refrigerant leaks, the equilibrium pressure is very low, and a low-pressure fault is reported when it is turned on. If the suction pressure is less than 0.2 MPa, it cannot be turned on. The computer displays "LPCURRENT", and the unit electronic board LP fault indicator lights up. Another possibility is that the refrigerant is sufficient, but the expansion valve is too small or blocked (or the refrigerant pipeline is not open), and it may cause a low pressure failure. In this case, the equilibrium pressure is often high, but the suction pressure is very low during operation, the discharge pressure is high, the compressor running current is also large, and the valve temperature is also very low. The expansion valve is frosted, and the pressure is long after shutdown. To restore balance. This situation generally occurs during low-temperature operation or at the beginning of each year. After a period of operation, it can return to normal.
(2) Insufficient flow of refrigerant water, less heat absorption, poor refrigerant evaporation effect, and supercooled and supersaturated steam, which is prone to wet compression. The pressure difference between inlet and outlet water becomes smaller, the temperature difference becomes larger, and the suction temperature is lower There is frosting on the suction port. The cause of insufficient water flow is: there is air or water in the system. The solution is to install an exhaust valve at a high place in the pipeline for exhausting; the pipeline filter is blocked or selected too thin, and the water permeability is limited. You should choose a suitable filter. The filter screen should be cleaned regularly; the pump should be smaller and not compatible with the system. Use a larger pump or use a spare pump.
(3) The evaporator is blocked, the heat exchange is poor, and the refrigerant cannot evaporate. The harm is the same as the lack of water. The difference is that the pressure difference between the inlet and outlet water becomes larger and frosting occurs on the suction port. Backwash.
(4) false alarms caused by electrical faults. False alarm caused by low voltage protection relay due to moisture short circuit, poor contact or damage, moisture or damage to the electronic board of the unit, communication failure.
(5) When the outside air temperature is low and the cooling water temperature is very low, low-pressure faults also occur. When the unit is running, due to insufficient pre-heating, low cooling oil temperature, and insufficient refrigerant separation, low-pressure faults also occur. . In the former case, measures such as closing the cooling tower and throttling the cooling water can be taken to increase the temperature of the cooling water. In the latter case, the warm-up time is extended, and the temperature of the frozen oil generally returns to normal after the temperature rises.

3. Low valve temperature failure The expansion valve outlet temperature reflects the evaporation temperature, which is a factor that affects heat transfer. Generally, the temperature difference between the outlet temperature of the refrigerant water and the refrigerant water is 5-6 ° C. When a low valve temperature fault occurs, the compressor will stop. When the valve temperature rises, it will automatically resume operation with a protection value of -2 ° C. The causes of low valve temperature failures are as follows:
(1) A small amount of refrigerant leakage, generally manifested as a low valve temperature fault rather than a low pressure fault. Insufficient refrigerant evaporates at the outlet of the expansion valve, causing cooling, which is manifested by frost at the expansion valve outlet, while the inlet temperature is higher (superheated steam), the cooling capacity is reduced, and the cooling is slow.
(2) The expansion valve is clogged or the opening is too small, and the system is not clean. For example, after the maintenance, the refrigerant pipeline is not cleaned, the refrigerant is not pure or the water content.
(3) Insufficient refrigerant water flow or blockage of the evaporator, poor heat exchange results in low evaporation temperature and low suction temperature, and the opening degree of the expansion valve is adjusted according to the suction temperature. If the temperature is low, the opening degree is small, resulting in Low valve temperature failure.
(4) False alarms caused by electrical faults, such as poor contact of the valve temperature line, cause the computer to display -5 ℃ unchanged.

4. Compressor overheat failure Thermistor is embedded in the compressor motor winding, and the resistance is generally 1kΩ. When the winding is overheated, the resistance value will increase rapidly. When it exceeds 141kΩ, the thermal protection module SSM will act and cut off the operation of the unit. At the same time, an overheating fault will be displayed and the TH fault indicator will be on. The causes of compressor overheating failure are as follows:
(1) The compressor is overloaded and operated with overcurrent. The possible reasons are: too high cooling water temperature, excessive refrigerant charge, or non-condensable gas such as air in the refrigeration system, resulting in a large load on the compressor, manifested as overcurrent, and accompanied by high voltage failure.
(2) Over-current operation of the compressor caused by electrical failure. For example, the three-phase power supply voltage is too low or the three-phase imbalance causes the current or the current of one phase to be too large; the AC contactor is damaged and the contacts are ablated, resulting in excessive contact current or excessive current due to lack of phase.
(3) The overheating protection module SSM is damp or damaged, the intermediate relay is damaged, and the contacts are bad. The overheating fault occurs when the power is turned on, and the compressor cannot start. If the unit's electronic board fails or communication fails, an overheating fault may also be falsely reported.

5. Communication failure

The computer controller controls each module through the communication line and the main interface board. The main cause of communication failure is the poor contact or disconnection of the communication line, especially the interface is affected by moisture and oxidation, and the unit electronic board or the main interface Board failure, improper selection of address dial switch, power failure can cause communication failure. Method of processing.



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