Commonly used cold dryers are divided into two types: air-cooled and water-cooled according to the cooling method of the condenser; according to the temperature of the intake air, there are high-temperature intake type (below 80 ℃) and normal temperature intake type (about 40 ℃); The pressure is divided into ordinary type (0.3-1.0MPa) and medium and high pressure type (above 1.2MPa).

Working principle and process

The refrigerated compressed air dryer makes the water vapor in the compressed air condense into droplets by cooling down, so as to achieve the purpose of reducing the moisture content. The condensed droplets are discharged from the dryer through the automatic drainage system. As long as the ambient temperature of the downstream pipeline of the dryer outlet is not lower than the dew point temperature of the evaporator outlet, there will be no secondary condensation.

air compression system

The purpose of the dryer is to dry the high pressure gas, reduce the temperature of the high pressure gas with a pressure of 30-32bar and a temperature of 30℃-50℃ to 1-6℃, filter out the water in it, and use it for the blow molding machine.

Cooling water system

The cooling water is mainly used to cool the refrigerant, turning the gaseous refrigerant into a liquid state, and the cooling water is cooled in the tower water machine.

Refrigerant system

Using the low boiling point characteristics of the refrigerant, it absorbs the temperature of the compressed air from liquid to gas during heat exchange, thereby reducing the temperature of the compressed air.

drainage system

The air is first cooled by the pre-cooler to remove a part of the water, and then greatly cooled by the evaporator to remove a large amount of water. The condensed water is drained to the sewage bucket through the water baffle or the steam-water separator, and then discharged through the drain.

Refrigeration principle and heat exchange principle

Refrigeration principle: When the system is working, the compressor sucks the low-pressure and low-temperature refrigerant vapor generated by the evaporator into the cylinder. The steam is discharged to the condenser. So the compressor plays the role of compressing and transporting the refrigerant vapor. In the condenser, the refrigerant vapor with higher temperature and pressure exchanges heat with air (or normal temperature water) with lower temperature and condenses into liquid. At this time, the liquid refrigerant is depressurized (cooled) by the expansion valve and then enters the evaporator, where it absorbs the heat of the compressed air and evaporates. In this way, the cooled object is cooled, and the refrigerant vapor is sucked away by the compressor to start the next cycle. In this way, the refrigerant completes a cycle through the four processes of compression, condensation, expansion (throttling), and evaporation in the system, and so on and on and on, and through the continuous refrigeration cycle, the purpose of refrigeration is achieved.

Heat transfer principle:

1. Sensible heat: Most heat transfer is a heat transfer method that relies on temperature difference to conduct heat.

2. Latent heat: The heat conducted by the phase transition of a substance at a constant temperature.

In the refrigeration system, most of the heat conduction is carried out in the form of latent heat conduction, and a small part is carried out in the form of sensible heat.

Main components of refrigeration system

1. Compressor

a. Function: compress low-temperature and low-pressure gaseous refrigerant into high-temperature and high-pressure superheated refrigerant vapor;

b. Main classification: piston type, scroll type, scraper type, screw type, centrifugal type;

c. Use occasions:

The power of the dryer is: 1-2 horses, heat pump, scraper type (for reference only);

The power of the cold dryer is: 3 to 12 horses, scroll type;

The power of the cold dryer is: 12 to 20 horses, (full) piston type;

The power of the cold dryer is: 25 to 65 horses, (semi) piston type;

The power of the cold dryer is: more than 65 horses, screw type.

2. Condenser

a. Function: After cooling and condensation, the superheated refrigerant vapor of high temperature and high pressure becomes a liquid refrigerant of high temperature and high pressure;

b. The condenser is divided into air-cooled and water-cooled. Determined according to the actual situation of the user

3. Expansion valve

a. Function: make the high temperature and high pressure liquid refrigerant into low temperature and low pressure liquid refrigerant;

b. The expansion valve is divided into internal balance type and external balance type. Attachment: Schematic

The working principle of the internal balance expansion valve:

When the internal balance expansion valve is working, its temperature sensor package is installed at the outlet of the evaporator to sense the superheat degree of the freon's return steam and perform automatic adjustment. After the refrigerant is throttled through the valve hole, it enters the evaporator at point A, and the refrigerant absorbs heat and evaporates to point B and all vaporizes into dry saturated steam. [Ignore the internal resistance of the evaporator] The temperature of the evaporator remains unchanged before point B, from point B At the beginning, the refrigerant is superheated steam, the temperature at point C is the temperature felt by the temperature sensor, and the pressure Ps in the temperature sensor is equal to the saturation pressure of the charging agent (R22, R12) corresponding to this temperature.

When the valve is in the equilibrium position: PS=P+Pf

Refrigerant R22

Therefore, the diaphragm pressure Ps=1bar (standard atmospheric pressure) when the temperature bulb is broken

The Ps﹤P﹢Pf expansion valve is closed, the refrigeration system is disconnected, and the high pressure trips.

4. Evaporator

a. Function: The refrigerant exchanges heat with the air, and the low-temperature and low-pressure liquid freon absorbs the heat in the air and turns into a low-temperature and low-pressure gas refrigerant and returns to the compressor;

b. Evaporator is divided into: shell and tube type, plate fin type.

Refrigeration accessories and functions

Other accessories of the refrigeration system include: filter drier, sight glass, hot gas bypass valve, liquid receiver, fan pressure switch and water volume control valve, vapor-liquid separator, high and low pressure protection switch, drainage device, etc.

1. Filter drier: filter the water and particulate impurities in the Freon system to remove the danger of acid formation.

2. Sight glass: a. Observe the flow of Freon and the state of the system; b. Observe whether there is water in Freon in the refrigeration system.

3. Water volume regulating valve (fan pressure switch): control the high pressure of the refrigerant in the refrigeration system, stabilize the high pressure, and ensure the high pressure.

4. Hot gas bypass valve (hand valve): control the low pressure of the refrigerant in the refrigeration system, that is, the evaporation temperature, to prevent ice blockage from freezing in the evaporator.

5. Liquid reservoir: ensure that the wind condenser has enough heat dissipation area.

6. Vapor-liquid separator: collect and store liquid refrigerant to ensure that only gaseous refrigerant enters the compressor to prevent the occurrence of liquid hammer.

7. High and low pressure protection switch: It performs the function of protecting the compressor itself to prevent the compressor motor from burning out. When the compressor discharge pressure exceeds the set value or the suction pressure is lower than the set value, the pressure control switch automatically cuts off the circuit, making the compressor stagnant and playing a safety protection role. normally:

High pressure setting value: 2.5Mpa

Low pressure setting value: 0.2Mpa

8. Fan pressure switch: Control the start and stop of the fan. Generally, the fan starts automatically when the high pressure is 1.4Mpa, and stops when the pressure drops to about 1.1~1.3Mpa. The difference with the high and low pressure switch is that the fan pressure switch has no reset device. The setting value is: 1.4Mpa.

The set value of the pressure switch has been adjusted in place when the dryer leaves the factory, and it is generally not necessary to adjust it.

Commonly used refrigerants for refrigerated dryers

Refrigerant R22:

a. Under standard atmospheric pressure (0.1Mpa), the boiling point is -40℃. When the gauge pressure (0.4Mpa), the boiling point is 0℃;

b. Commonly used medium temperature refrigerants;

c. Colorless, odorless, non-flammable, non-explosive, and a safe refrigerant;

d. Can partially dissolve with lubricating oil, and its solubility varies with the type and temperature;

e. Moisture content 0.06%.

1. Refrigerant R22

2. Refrigerant R134a

Installation requirements for the refrigerated dryer

1. The refrigerated dryer should be installed in a building with an ambient temperature of 2-45 °C, good ventilation and clean air; especially, avoid corrosive components (such as ammonia) in the air. When the ventilation conditions of the indoor equipment room are poor, ventilation and exhaust equipment must be installed in the equipment room.

2. There is a distance of at least 4-5 meters between the air dryer and the air compressor to prevent the vibration generated by the air compressor from affecting the normal operation of the air dryer.

3. The distance between the air inlet of the air-cooled dryer and the wall should be more than 1.5m, and the air inlet and exhaust port of the two dryers should not face each other. There should be a certain space around the dryer to facilitate maintenance and daily maintenance.

4. If the cooling water quality of the water-cooled dryer is poor, a cooling water filter should be installed at the entrance to filter impurities. The cooling water temperature is better below 32oC, and the water pressure should be kept between 0.15-0.50MPa.

5. When the dryer is running, the vibration itself is very small, and the cement foundation is only needed in the soft ground, and the foundation needs to be flat. The vibration of the air compressor from the pipeline should be avoided. If it is too serious, a high-pressure hose should be connected to eliminate the vibration.

6. The outlet of the automatic drain should be connected directly to the drain pipe to the drain, so as not to pollute the production site, and the height of the drain should be lower than the outlet of the automatic drain.

7. The refrigerating machine is connected to the air source system. It is best to connect a bypass pipeline between the air inlet and the outlet, and set a stop valve in the middle for debugging, maintenance and start-up.

Preparations and commissioning before start-up

(1) Do the preparatory work before start-up

1. Open the electric control box and fasten all terminals to avoid loosening during transportation and installation; check whether the power supply voltage is consistent with the nameplate, and the voltage fluctuation range cannot exceed 10% of the rated voltage;

2. Check the evaporating pressure gauge and condensing pressure gauge on the panel. The display of the two pressure gauges should be balanced, and the reading should not be less than 0.4~0.55Mpa; It should be reported to the service department immediately for refrigeration system inspection;

3. Check the working conditions: whether the air inlet pressure, inlet temperature and processing air volume, cooling water pressure and temperature are consistent;

4. For water-cooled dryers, open the cooling water inlet and outlet valves;

5. The compressed air piping system must be purged before starting the machine to prevent debris from entering the dryer and filter and affecting the use.

Method: Cut off the compressed air inlet valve and outlet valve of the drying and purification equipment, open the air bypass valve to start the air compressor, and let the gas pass the bypass for about half an hour for purging.

(2) No-load debugging

1. Reset the low-voltage protection value to 0.1Mpa;

2. Close the hot gas bypass valve and the hand valve, so that the freon is fully throttled through the expansion valve;

3. Observe that the evaporation pressure should be kept between 0.18-0.3 Mpa (determined by the opening degree of the expansion valve), make the surface of the compressor freeze, and run for about 5 minutes;

4. Slowly adjust the hot gas bypass valve so that the evaporation pressure slowly moves up to 0.4Mpa, then the compressor starts to defrost; the compressor temperature is about 10°C, and the exhaust temperature is about 50-80°C.

Note: If it is found that the compressor has abnormal noise and the pressure gauge changes abnormally when starting up, it should be stopped immediately to check and troubleshoot in time.

(3) Loading and debugging

The no-load operation is normal for about half an hour, and the loading and debugging are entered.

1. Close the air outlet valve, slowly open the compressed air inlet valve (especially for small refrigerated dryers), and keep the air bypass valve open (if there is an air bypass system) to slowly pressurize the dryer;

2. Check the air pipes, flanges and drains for air leakage, and troubleshoot in time;

3. After running for about 15 minutes, slowly open the air outlet valve and close the bypass valve;

4. Adjust the hot gas bypass valve (hand valve) and the expansion valve of the small air dryer; make the evaporation pressure within the range of 0.4~0.55 Mpa, observe and record the fluctuations of the condensation pressure and evaporation pressure, and check whether the action of the drainage device is normal. Observe the fan operation interval and compare it with no-load operation. Readjust the pressure switch setting value to 0.2Mpa.

5. Adjust the condensing pressure, generally it should be 1.2~1.6 Mpa (the maximum is 2.5 Mpa); if it is lower than this range, for the air-cooled type, check and adjust the setting value of the high and low pressure switch of the fan; for the water-cooled type, adjust the water volume control valve the opening size.

Note: When loading and debugging, the evaporation pressure will fluctuate a little accordingly.

It is a normal phenomenon that the cooling fan is always on and off, and it should be determined according to the working conditions.

◆ Hot gas bypass valve adjustment method: Rotate the bypass valve adjustment screw clockwise, the evaporation pressure will increase; otherwise, the evaporation pressure will decrease.

◆ Expansion valve adjustment method (small refrigerated dryer): unscrew the bulky head at the lower end of the expansion valve, rotate the valve stem 1/4-1/2 turn each time, the evaporating pressure will increase clockwise; otherwise, the evaporating pressure will decrease;

Note: 1. The expansion valve generally does not need to be adjusted. Only when the hot gas bypass valve cannot be adjusted, first close all the hot gas bypass valves, adjust the expansion valve to stabilize the evaporation pressure at 0.3 Mpa, and then adjust the hot gas bypass valve.

2. The adjustment of the three valves is not independent, but interrelated; when adjusting, it is necessary to adjust the valve while observing the pressure change until the working pressure is within the normal range and stable.

(4) Shutdown steps: before shutting down, manually drain (or drain) until it is clean; when shutting down, first close the air inlet valve, then press the stop button to cut off the power supply; for water-cooled type, close the cooling water inlet valve.