Introduction to Lithium Bromide Absorption Chillers

2025-01-02

Absorption refrigerators are a type of water refrigeration unit that uses water as the refrigerant and lithium bromide as the absorbent.
Refrigeration is achieved through the evaporation of the refrigerant; the refrigerator operates in a high vacuum state, so it is necessary to ensure the vacuum degree of the unit; the absorption liquid (lithium bromide aqueous solution) has a strong water absorption property.
Absorption chillers are cooling and heating water generating devices that use water (H₂O) as the refrigerant and lithium bromide (LiBr) solution as the absorbent.
It takes 100 Kcal of heat (sensible heat) to heat 100g of water from 0°C to 100°C, and it takes 540 Kcal of heat (latent heat of vaporization) to evaporate 100g of 100°C water into 100°C steam.
Latent heat > sensible heat. Water evaporates at 100°C under normal pressure (760 mmHg). When the pressure is only 1/100 of atmospheric pressure (absolute pressure 6 mmHg), water can evaporate at 4°C. Our refrigeration unit uses water evaporation to produce refrigerant water. Place the refrigerant water in a sealed container to make the container close to a vacuum state (6 mmHg), and then the water will evaporate at 4°C. We let the cold water pass through the container to absorb heat, and then 7°C cold water (refrigerant water) can be produced - the container is called an evaporator.
Evaporation is a heat-absorbing process - the refrigerant water evaporates to absorb the heat of the refrigerant water to achieve the purpose of refrigeration.
The evaporated refrigerant vapor should be discharged outside the evaporator to ensure the continuation of the refrigeration process. Therefore, it is necessary to connect a container filled with a strongly absorptive substance to absorb the evaporated refrigerant vapor and ensure that the pressure in the container is 6 mmHg.
The LiBr solution has a strong absorption property. The higher the concentration of the solution and the lower the temperature, the stronger its absorption property. We use the lithium bromide (LiBr) aqueous solution as an absorbent. The refrigerant vapor is absorbed in the container, and this container is called an absorber.
However, when the refrigerant evaporated at 4°C is absorbed by the absorption liquid, the absorption liquid will release absorption heat, the temperature of the absorption liquid will rise, and the absorption capacity will decrease.
Therefore, cooling water is used for cooling to prevent the decrease of absorption capacity. This absorption heat is equivalent to the latent heat of vaporization of the refrigerant, that is, the heat of the cold water is transferred to the refrigerant vapor through the evaporation of the refrigerant, the refrigerant vapor is absorbed into the absorber, and the heat released is cooled and then transferred to the cooling water.
The concentrated lithium bromide solution becomes a dilute solution due to the absorption of refrigerant vapor, thereby losing its absorption capacity. How to turn the dilute lithium bromide solution back into a concentrated solution?
The dilute lithium bromide solution is pumped by the solution pump into the generator. Under the heating of an external heat source, the dilute lithium bromide solution becomes a concentrated solution. At the same time, refrigerant vapor is generated.
The concentrated lithium bromide solution generated by heating restores its absorption capacity, flows back to the absorber, and continues to absorb the refrigerant vapor from the evaporator.
The temperature of the concentrated lithium bromide solution is relatively high, and the dilute lithium bromide solution needs to be heated. In order to make full use of energy, we set up a heat exchanger in the process of the concentrated lithium bromide solution flowing back from the generator to the absorber and the dilute lithium bromide solution being pumped from the absorber to the generator, so that the two can exchange heat.
When the refrigerant vapor generated in the generator reaches saturation, the dilute lithium bromide solution can no longer evaporate. How to handle the refrigerant vapor generated in the generator?
Generator: When the absorption liquid absorbs the refrigerant vapor, the absorption capacity decreases with the decrease of the concentration. To restore the absorption capacity, the absorption liquid is pumped into another container to be heated to separate the refrigerant vapor - the generator (the absorption liquid is concentrated after evaporating the refrigerant vapor and then sent back to the absorber to complete the solution cycle and continue the refrigeration).
The function of the condenser is:
The refrigerant vapor from the generator is condensed into liquid water by cooling the cooling water in the copper pipe of the condenser.
The liquid water flows back to the evaporator to continue to evaporate and absorb heat, so that the refrigerant water in the evaporator is continuously replenished. Thus, a complete refrigeration cycle is completed.
The cooling water enters the cooling tower from the outlet, and under the action of the cooling tower fan, the heat in it is dissipated into the atmosphere, the temperature drops to 32°C, and then enters the refrigerator from the cooling water inlet. This cycle repeats continuously.
The refrigerant water comes out of the refrigerator and then enters the air conditioner (or fan coil) to send the cooling capacity to the position where refrigeration is required.
The unit is a combination of multiple exchangers:
Specifically, the unit consists of an evaporator, an absorber, a high-pressure (temperature) generator, a low-pressure (temperature) generator, a condenser, a high-low temperature heat exchanger, a refrigerant water pump, a dilute solution pump, a concentrated solution pump, a vacuum pump, a condensate heat exchanger, a condensate drainer, a steam regulating valve, and an automatic air extraction device.

Evaporator: The evaporator is the place where the unit produces cold (hot) water. It is a shell-and-tube heat exchanger with a spray-type structure inside, and the heat exchange tubes are high-efficiency heat exchange tubes. The refrigerant water is sprayed by the refrigerant pump onto the outer surface of the heat exchange tubes and continuously evaporates to absorb the heat of the circulating water in the tubes, reducing its temperature. The main components include tube sheets, heat transfer tubes, support plates, spray headers, and nozzles.
Absorber: The absorber is the same as the evaporator, also a shell-and-tube heat exchanger with a spray-type structure inside, and the heat exchange tubes are copper bare tubes. The refrigerant vapor from the evaporator passing through the liquid baffle is absorbed by the sprayed concentrated solution, the concentrated solution becomes a dilute solution, and heat is released at the same time. The heat is taken away by the cooling water flowing in the heat exchange tubes. The main components include tube sheets, heat transfer tubes, support plates, spray headers, nozzles, and air extraction headers.
High-pressure (temperature) generator The high-temperature generator is a very critical component in the absorption refrigerator, usually made into a single unit. It mainly consists of a cylinder, tube sheets, heat transfer tubes, etc.
Low-pressure (temperature) generator: The low-temperature generator is also a shell-and-tube heat exchanger, and the internal structure of the low-temperature generator is of a spray type. The dilute solution is sprayed onto the outer surface of the heat exchange tubes, and the refrigerant vapor generated by the high-temperature generator flows in the heat exchange tubes to heat the dilute solution, and at the same time flows to the condenser together with the generated refrigerant vapor. The main components include tube sheets, heat transfer tubes, support plates, spray headers, and nozzles.
Condenser: The condenser is also a shell-and-tube heat exchanger. The refrigerant vapor from the generator condenses into refrigerant water on the surface of the heat exchange tubes, and the released heat is taken away by the cooling water flowing in the heat exchange tubes. The main components include tube sheets, heat transfer tubes, and support plates.

Classification of lithium bromide absorption refrigerators:

According to the application: 1) Chiller unit; 2) Cooling and heating unit; 3) Heat pump unit.
According to the driving heat source: 1) Steam type; 2) Direct-fired type; 3) Hot water type.
According to the utilization mode of the driving heat source: 1) Single-effect; 2) Double-effect; 3) Multi-effect.
According to the solution circulation process:

Series process, which is divided into two types. One is that the solution first enters the high-pressure generator, then enters the low-pressure generator, and finally flows back to the absorber;
The other is that the solution first enters the low-pressure generator, then enters the high-pressure generator, and finally flows back to the absorber.
Parallel process, the solution enters the high and low-pressure generators respectively and simultaneously, and then flows back to the absorber respectively;
Series-parallel process, the solution enters the high and low generators respectively and simultaneously. The solution flowing out of the high-pressure generator first enters the low-pressure generator, and then flows back to the absorber together with the solution of the low-pressure generator.

According to the unit structure:

Single-cylinder type, the main heat exchangers (generator, condenser, evaporator, absorber) of the unit are arranged in one cylinder.
Double-cylinder type, the main heat exchangers of the unit are arranged in two cylinders.
Three-cylinder or multi-cylinder type, the main heat exchangers of the unit are arranged in three or more cylinders.

Evaporator:
In the evaporator: Water evaporates at 4°C under the condition of 6 mmHg.
In the absorber: The refrigerant vapor of the evaporator is absorbed by the concentrated lithium bromide solution in the absorber, and the solution concentration becomes thinner. (During the evaporation process of the evaporator, it absorbs the heat of the refrigerant water, making the refrigerant water become 7°C - 12°C refrigerant water.)
High-pressure generator:
The dilute solution in the absorber is introduced into the generator by the solution pump, and the solution is concentrated by steam heating, the concentration becomes thicker, the concentrated solution returns to the absorber to absorb the refrigerant water, and the evaporated refrigerant vapor is condensed by the cooling water and condensed into refrigerant water and returns to the evaporator.
Low-pressure generator: It consists of copper pipes, liquid baffles connecting to the condenser, etc. The intermediate solution from the high generator exchanges heat with the refrigerant vapor inside the copper pipes on the outside of the copper pipes. The intermediate solution is evaporated and concentrated into a concentrated solution - the concentrated solution passes through the low-temperature heat exchanger to the absorber. The refrigerant vapor is sent to the condenser in the condenser (condensed into refrigerant water in the condenser and enters the condenser liquid reservoir).
Condenser: It has copper pipes and liquid baffles. The condenser uses cooling water and refrigerant vapor from the low generator, and the condensed refrigerant water flows into the evaporator.
High-low temperature heat exchanger: They are mainly composed of copper pipes. The dilute solution flows through the tube side, and the intermediate solution or concentrated solution flows through the shell side, mainly to improve the thermal efficiency. As mentioned earlier, ① the absorbent is more conducive to absorption at low temperatures, so it is necessary to cool the absorbent generated by the high generator as much as possible. ② On the other hand, the higher the temperature of the solution, the more conducive it is to separate the refrigerant vapor from the solution.
Therefore, a heat exchanger is set up to use the dilute solution to reduce the temperature of the intermediate and concentrated solutions.

I. Inspection items before operation:

On the microcomputer control panel, switch the operation of the cold water pump and the cooling water pump to the automatic state (except for manual operation).
If the manual light of the control valve on the current control panel is on, it means that the control valve is in the manual mode. Press the control valve switch again to switch to the automatic mode.
Check whether the control valve is normally open and check the pressure of the steam pipeline.
Check whether the valves of the refrigerant water pipeline are normally open, whether the refrigerant water pump can be normally started, and whether the water pressure is normal; check whether the valves of the cooling water pipeline are normally open, whether the cooling water pump is normally started, and whether the water pressure is normal (when operating the refrigerant water pump and the cooling water pump, check the opening state of the valves again).
Check and confirm that the cooling tower cooling fan unit is normally started (it is not started in the automatic state).
After checking the above items, the operator can enter the normal operation of the unit only after confirming that they are normal.

II. Startup (automatic operation):

Press the operation switch on the air conditioner control panel for 3 seconds, and the system will enter the startup state. In the remote operation state, open the remote switch (computer).
When the air conditioner enters the operation state, check the cold water pump and the cooling water pump. To ensure the normal circulation of cooling water and cold water, check the water pressure of each part. When the outlet pressure of the cold water pipe and the cooling water pipe is higher than the inlet pressure, it is in a normal state. If the inlet and outlet pressures are the same or the pressure difference decreases, quickly stop the air conditioner and switch the cold water pump to the manual state to find the cause and handle the problem. If the temperature pointer swings severely or the pressure difference is large, this is mainly caused by insufficient water pressure in the pipeline. Check the cold water and cooling water pipelines and the system water volume to supply water. (Special note: Because the air conditioner in the automatic working state starts under normal conditions, even if the cooling water volume is insufficient or the cooling tower fan is abnormal, the unit will continue to operate. This needs special attention.
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Check whether the pressure of the steam pipeline is normal and whether the cooling tower fan is operating normally.

III. Shutdown (automatic operation):

Press the stop key on the control panel for 3 seconds.
Automatically close the steam input control valve within one minute.
The cooling water pump stops working.
The chilled water pump stops working.
The dilution operation during shutdown takes about 5 - 15 minutes, and the stop time of the cold water pump and the cooling water pump is determined by the temperature of the high generator.

IV. Manual operation:

Manually start the cold water pump and the cooling water pump, check that the circulation of the cold water pump and the cooling water pump is normal, press the start key on the air conditioner control panel for 3 seconds, and the unit will start the steam automatic operation.
Check whether the steam and the cooling tower fan are normal.

V. Manual shutdown:

Press the stop key on the control panel for 3 seconds.
Stop supplying steam within one minute.
After the dilution operation for 5 - 15 minutes, the air conditioner stops running.
Stop the cooling water pump.
Stop the cold water pump.
Stop the cooling tower (automatic or manual) fan.

VI. Special attention:

When the operator forgets to close the air extraction valve or air enters the unit due to abnormal operation, the vacuum of the unit is damaged. It is necessary to immediately check the cause and completely remove the air.
Once steam leaks, immediately close the steam inlet valve and repair it.
During the dilution operation of the unit, do not close the cold water pump and the air conditioner until the dilution operation is completely finished. The sudden stop of the air conditioner will cause the danger of overcooling. Especially during operation, try not to manually operate the cold water pump as much as possible. Because during the dilution operation when the unit stops operating and the cooling water continues to flow, the water that cannot flow in the copper pipe of the evaporator will be overcooled by the residual cold in the unit.
Do not use a megohmmeter to measure the temperature regulation control elements on the motherboard.
The cold water pump and the cooling water pump operate automatically through the interlock cycle, that is to say, the automatic operation is effective only when the cold water pump, the cooling water pump, and the cooling tower are effectively interlocked. Therefore, it is necessary to check and ensure that their interlock is normal.
The start-up and shutdown procedures of the interlock operation are as follows:
① Start-up procedure: Cold water pump → Cooling water pump → Cooling tower → Unit;
② Shutdown procedure: Unit → Cooling water pump → Cooling tower → Cold water pump.

Daily maintenance of lithium bromide refrigerators:
The lithium bromide absorption refrigerator is based on the change of the basic state parameters of the fluid and the theory of the heat transfer process of substances, and uses the characteristics of the lithium bromide binary solution and its thermodynamic state change law to carry out the refrigeration cycle. It has high requirements for the vacuum degree of the unit. Usually, it is necessary to carry out daily maintenance on the unit. The main contents are as follows: (This paragraph takes LS air conditioner as an example)

Short-term shutdown maintenance When the shutdown time is within 1 - 2 weeks, the maintenance work mainly focuses on maintaining the vacuum degree of the unit. The vacuum degree should be monitored twice a day, in the morning and evening. For accuracy, start the generator pump and the absorber pump to run for 10 minutes before observing the pressure gauge, and then observe the instrument reading and compare it with the previous one.
Long-term shutdown maintenance For long-term shutdown, all the refrigerant water in the evaporator should be bypassed to the absorber, and the solution should be evenly diluted to prevent crystallization at the ambient temperature. There is no unified regulation for the maintenance method during the shutdown period. Generally, two maintenance methods, vacuum and nitrogen filling, are adopted.
Nitrogen filling maintenance is to fill the unit with nitrogen at about 49 kPa (gauge pressure) when it is ensured that the unit has no leakage, so that it is always in a positive pressure state. Even if there is a leakage in the unit, air will not leak in, and if there is a leakage, it can be detected at any time, which is very convenient. Its disadvantages are: due to the relatively complex structure and process of the unit, it is difficult to remove nitrogen at one time.
When starting the machine, the refrigeration efficiency cannot meet the requirements, and it is necessary to continue to start the vacuum pump to evacuate. In addition, it also requires the funds to purchase nitrogen.
Vacuum maintenance requires that the unit maintain a relatively high vacuum degree after shutdown. This method is relatively simple, not only saves expenses, but also saves the nitrogen filling process operation. If the vacuum degree is still qualified before the unit is trial-run, it can be directly started and put into operation. Vacuum maintenance also has disadvantages: once the monitoring is not strict or the analysis is incorrect, air will leak in and cause corrosion. In addition, if the refrigerator leaks due to poor sealing quality, it is necessary to fill nitrogen to increase the pressure for leak detection. Therefore, after shutdown, it is more proactive to fill nitrogen immediately after shutdown than to wait for leakage to occur