Do You Know All These Knowledges about the Absorption Unit of the Lithium Bromide Refrigeration Machine?

2025-03-31

The absorption refrigerator is a water refrigeration unit that uses water as the refrigerant and lithium bromide as the absorbent.

It refrigerates through the evaporation of the refrigerant. The refrigerator operates in a high-vacuum state, so the vacuum degree of the unit must be ensured. The absorption liquid (lithium bromide aqueous solution) has a strong water absorption capacity.

The absorption chiller is a device for generating cooling and heating water that uses water (H₂O) as the refrigerant and a 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 water at 100°C into steam at 100°C.

The latent heat is greater than the sensible heat. Water evaporates at 100°C under normal pressure (760 mmHg). When the pressure is only 1/100 of an atmosphere (absolute pressure of 6 mmHg), water can evaporate at 4°C. Our refrigeration unit uses the evaporation of water to produce chilled water. Place the refrigerant water in a sealed container to make the inside of the container close to a vacuum state (6 mmHg), and at this time, the water evaporates at 4°C. We let the cold water pass through the container and be absorbed of heat, and then we can produce 7°C cold water (chilled water) - the container is called the evaporator.

Evaporation is a process of absorbing heat - the refrigerant water evaporates and absorbs the heat of the chilled water to achieve the purpose of refrigeration.

The evaporated refrigerant vapor should be discharged outside the evaporator to ensure that the refrigeration process continues. Therefore, it is necessary to connect a container filled with a substance with strong absorbency to absorb the evaporated refrigerant vapor and ensure that the pressure inside the container is 6 mmHg.

The LiBr solution has a strong absorbency. The higher the concentration of the solution and the lower the temperature, the stronger its absorbency. We use the lithium bromide (LiBr) aqueous solution as the absorbent. The refrigerant vapor is absorbed in the container, and this container is called the absorber.

However, when the refrigerant evaporated at 4°C is absorbed by the absorption liquid, the absorption liquid will release the heat of absorption, the temperature of the absorption liquid will rise, and the absorbency will decrease.

Therefore, cooling water is used for cooling to prevent the absorbency from decreasing. This heat of absorption 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 by it is transferred to the cooling water after being cooled.

The concentrated lithium bromide solution becomes a dilute solution because it absorbs the refrigerant vapor, and thus loses its absorbency. How can the dilute lithium bromide solution be changed back to 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 absorbency and flows back to the absorber to continue absorbing 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 install 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 conduct heat exchange.

When the refrigerant vapor generated in the generator reaches saturation, the dilute lithium bromide solution can no longer evaporate. How to deal with the refrigerant vapor generated in the generator?

Generator: When the absorption liquid absorbs the refrigerant vapor, the absorbency decreases as the concentration decreases. To restore the absorbency, the absorption liquid is pumped into another container to be heated to separate the refrigerant vapor - the generator (after the absorption liquid evaporates the refrigerant vapor, it is concentrated and sent back to the absorber, completing the solution cycle and enabling the refrigeration to continue).

The function of the condenser is:

The cooling water in the copper tubes of the condenser cools down, and the refrigerant vapor from the generator is condensed into liquid water.

The liquid water flows back to the evaporator to continue evaporating and absorbing heat. This continuously replenishes the refrigerant water in the evaporator. At this point, a complete refrigeration cycle is completed.

The cooling water enters the cooling tower from the outlet. Under the action of the cooling tower fan, the heat in it is dissipated into the atmosphere, and the temperature drops to 32°C, and then it enters the refrigerator from the cooling water inlet. This cycle repeats.

After the chilled water comes out of the refrigerator, it enters the air conditioner (or fan coil unit) to deliver the cooling capacity to the location 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 pump, a dilute solution pump, a concentrated solution pump, a vacuum pump, a condensate heat exchanger, a condensate drain, a steam regulating valve, and an automatic air extraction device.

Evaporator: The evaporator is the place where the unit produces cold (warm) water. It is a shell-and-tube heat exchanger with a spray 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, absorbing the heat of the circulating water in the tubes and 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. It is also a shell-and-tube heat exchanger with a spray structure inside, and the heat exchange tubes are bare copper tubes.

The refrigerant vapor coming from the evaporator through the liquid baffle is absorbed by the sprayed concentrated solution. The concentrated solution turns into a dilute solution and releases heat simultaneously. The heat is carried away by the cooling water flowing inside the heat exchange tubes. The main components include tube sheets, heat transfer tubes, support plates, spray headers, nozzles, and an air extraction header.

High-pressure (temperature) generator: The high-temperature generator is a crucial component of the absorption refrigerator, usually constructed as a single unit. It is mainly composed of a cylinder body, tube sheets, heat exchange tubes, etc.
Low-pressure (temperature) generator: The low-temperature generator is also a shell-and-tube heat exchanger with a spray structure inside.
The dilute solution is sprayed onto the outer surface of the heat exchange tubes. The refrigerant vapor generated by the high-temperature generator flows inside the heat exchange tubes, heating the dilute solution, and simultaneously flowing towards 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 carried away by the cooling water flowing inside the heat exchange tubes. The main components include tube sheets, heat transfer tubes, and support plates.

Classification of lithium bromide absorption refrigerators:

Classification by purpose: 1) Chiller units; 2) Chilled and hot water units; 3) Heat pump units.
Classification by driving heat source: 1) Steam type; 2) Direct-fired type; 3) Hot water type.
Classification by the utilization method of the driving heat source: 1) Single-effect; 2) Double-effect; 3) Multi-effect.
Classification by solution circulation process:

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

Classification by unit structure:

Single-cylinder type, where the main heat exchangers (generator, condenser, evaporator, absorber) of the unit are arranged within one cylinder.
Double-cylinder type, where the main heat exchangers of the unit are arranged in two cylinders.
Three-cylinder or multi-cylinder type, where 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 from the evaporator is absorbed by the concentrated lithium bromide solution in the absorber, and the concentration of the solution becomes thinner. (During the evaporation process in the evaporator, the refrigerant water absorbs the heat of the chilled water, turning the chilled water into chilled water at a temperature range of 7°C to 12°C)

High-pressure generator:
The dilute solution in the absorber is pumped into the generator by the solution pump. The solution is concentrated by steam heating, and its concentration becomes higher. The concentrated solution returns to the absorber to absorb the refrigerant water. The refrigerant vapor separated by evaporation is condensed by the cooling water and turns into refrigerant water, which then returns to the evaporator.

Low-pressure generator: It is composed of copper tubes, the liquid baffle connected to the condenser, etc. The intermediate solution from the high-pressure generator exchanges heat with the refrigerant vapor inside the copper tubes on the outside of the copper tubes. The intermediate solution undergoes evaporation and concentration to become 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 (it condenses into refrigerant water in the condenser and enters the liquid pocket of the condenser).

Condenser: It has copper tubes and a liquid baffle. The condenser uses the cooling water and the refrigerant vapor from the low-pressure generator. The condensed refrigerant water flows into the evaporator.

High-low temperature heat exchanger: They are mainly composed of copper tubes. The dilute solution flows through the tube side, and the intermediate solution or the concentrated solution flows through the shell side, mainly aiming to improve the thermal efficiency. As mentioned before, ① the absorbent is more conducive to absorption at a lower temperature, so try to cool down the absorbent generated by the high-pressure generator as much as possible. ② On the other hand, the higher the temperature of the solution, the more conducive it is to separating the refrigerant vapor from the solution.
Therefore, a heat exchanger is installed to use the dilute solution to lower the temperature of the intermediate and concentrated solutions.

Single-effect absorption chiller:
Double-effect absorption chiller:
See the following principle dynamic diagram:
Working flow chart:
Comparison between the physical structure and the process:
Safety operation procedures and maintenance:
Ⅰ Inspection items before operation:

On the microcomputer control panel, switch the operation of the chilled water pump and the cooling water pump to the automatic state (except for manual operation).
When the manual indicator light of the control valve on the current control panel is on, it indicates 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 opened normally and check the pressure of the steam pipeline.
Check whether the valves of the chilled water pipeline are opened normally, whether the chilled water pump can be started normally, and whether the water pressure is normal; check whether the valves of the cooling water pipeline are opened normally, whether the cooling water pump is started normally, and whether the water pressure is normal (when operating the chilled water pump and the cooling water pump, check the opening status of the valves again).
Check and confirm that the cooling fan unit of the cooling tower is started normally (it will not start in the automatic state).
After checking the above items, the operator can enter the normal operation of the unit only after confirming that everything is 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, turn on the remote switch (computer).
When the air conditioner enters the operation state, check the chilled water pump and the cooling water pump. To ensure the normal circulation of the cooling water and the chilled water, check the water pressure of each part. It is in a normal state only when the outlet pressure of the chilled water pipe and the cooling water pipe is higher than the inlet pressure. If the inlet and outlet pressures are the same or the pressure difference decreases, stop the air conditioner immediately and switch the chilled water pump to the manual state to find the cause and solve the problem. If the temperature pointer swings violently or the pressure difference is large, this is mainly caused by insufficient water pressure in the pipeline. Check the chilled water and cooling water pipelines, as well as the water volume of the system and replenish water. (Special note: Since 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 not working properly, the unit will continue to operate. This point requires special attention.)
Check whether the pressure of the steam pipeline is normal and whether the cooling tower fan is operating normally.
Ⅲ Shutdown (Automatic Operation)
Hold down the stop button 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 to 15 minutes, and the stop time of the chilled water pump and the cooling water pump is determined by the temperature of the high-pressure generator.
Ⅳ Manual Operation
Manually start the chilled water pump and the cooling water pump, check that the circulation of the chilled water pump and the cooling water pump is normal, and press the start button on the air conditioner control panel for 3 seconds, and the unit will start the automatic operation of steam.
Check whether the steam and the cooling tower fan are normal.
Ⅴ Manual Shutdown
Press the stop button on the control panel for 3 seconds.
The steam supply stops within one minute.
After the dilution operation for 5 to 15 minutes, the air conditioner stops operating.
Stop the cooling water pump.
Stop the chilled 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, and the cause must be checked immediately and the air must be completely removed.
Once there is a steam leak, immediately close the steam inlet valve and repair it.
During the dilution operation of the unit, close the chilled water pump and the air conditioner only after the dilution operation is completely finished. The sudden stop of the air conditioner will pose a risk of overcooling. Especially during the operation period, try not to operate the chilled water pump manually as much as possible. Because when the unit stops the dilution operation and the cooling water continues to flow, the water that cannot flow in the copper tubes of the evaporator will be excessively cooled by the residual cooling capacity left in the unit.
Do not use a megohmmeter to measure the temperature regulation control components on the main board.
The chilled water pump and the cooling water pump are automatically operated through an interlock cycle, that is to say, the automatic operation is effective only when the chilled water pump, the cooling water pump, and the cooling tower are effectively interlocked. Therefore, it is necessary to check and ensure that the interlock is normal. The starting and closing procedures of the interlock operation are as follows:
① Starting procedure: Chilled water pump → Cooling water pump → Cooling tower → Unit;
② Closing procedure: Unit → Cooling water pump → Cooling tower → Chilled water pump.
Daily Maintenance of Lithium Bromide Refrigerators
The lithium bromide absorption refrigerator is based on the theory of the changes in the basic state parameters of the fluid and the heat transfer process of substances, and uses the characteristics of the lithium bromide binary solution and the laws of its thermodynamic state changes to carry out the refrigeration cycle. It has a very high requirement for the vacuum degree of the unit. Daily maintenance must be carried out on the unit at ordinary times, and the main contents are as follows: (This paragraph takes the LS air conditioner as an example)
Short-term Shutdown Maintenance When the shutdown time is within 1-2 weeks, the maintenance work is mainly to maintain 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 and let them 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 uniformly diluted to prevent crystallization at the ambient temperature. There is no unified regulation on the maintenance method during the shutdown period. Generally, two maintenance methods, vacuum maintenance and nitrogen charging maintenance, are used.
The nitrogen charging maintenance is to charge the unit with nitrogen at about 49kPa (gauge pressure) when it is ensured that there is no leakage in the unit, 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 the leakage can be detected at any time, which is very convenient. Its disadvantage is that due to the relatively complex structure and process of the unit, it is difficult to remove the nitrogen at one time. When starting up, 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 funds for purchasing nitrogen.
The vacuum maintenance is to keep a relatively high vacuum degree in the unit after the unit is shut down. This method is relatively simple, not only saving expenses but also eliminating the nitrogen charging 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 its drawbacks. Once the monitoring is not strict enough or there is an analytical error, air may leak in, causing corrosion. Additionally, if the refrigerator leaks due to poor sealing quality, nitrogen must be charged to increase the pressure for leak detection. Therefore, it is more proactive to charge nitrogen immediately after shutdown rather than waiting for a leak to occur before charging nitrogen for treatment. Of course, for refrigerators with excellent sealing quality, the situation is different. For vacuum maintenance, a dedicated person must be assigned to monitor the vacuum degree of the unit twice a day. Any leakage found should be dealt with immediately, and no delay is allowed.

Internal Cleaning of the Lithium Bromide Refrigerator:
The cleaning of the lithium bromide refrigerator in the central air conditioning system includes internal cleaning and system cleaning.

Internal Cleaning of the Lithium Bromide Refrigerator
The chemical cleaning of the lithium bromide solution circulation system is a measure taken when the internal corrosion of the unit is severe and the unit can no longer operate normally. It is the only way to clean the inner cavity of the unit, and it is generally carried out once every 4 to 5 years. Through cleaning, the rust generated by corrosion in the inner cavity of the unit can be completely removed, the heat transfer effect of the inner cavity can be improved, and the spraying effect can be enhanced. The newly filled solution will not be affected by impurities and can exert the best refrigeration capacity in the best state. By pre-filming the inner cavity wall of the unit, the pre-filming agent undergoes a chemical reaction on the surface of the material, forming an inert protective film, thus reducing the corrosion of the unit and prolonging its service life.
Cleaning of the Cooling Water and Chilled Water System of the Lithium Bromide Refrigerator
During the long-term circulation process, a hard layer of dirt and rust will form on the inner walls of copper pipes, pipelines, etc. Sometimes, it may even cause pipeline blockages, seriously affecting the heat exchange between heat and mass and resulting in a significant decrease in the refrigeration capacity of the unit. Therefore, the water circulation system must be cleaned regularly. This cleaning includes the cleaning of copper pipes and the water system during the winter maintenance of the unit.
Regeneration Treatment of the Lithium Bromide Solution
The lithium bromide solution is the "blood" of the unit, and it will change to varying degrees after long-term operation. For example, its color may change from the original light yellow to abnormal colors such as dark yellow, red, white, and black. The concentration of the solution decreases due to corrosion products, the pH value of the solution becomes strongly alkaline or acidic, the corrosion inhibitor in the solution fails, and the content of various impurity ions increases. All these will lead to the inability of the unit to fully exert its normal refrigeration capacity and the aggravation of the corrosion of the unit itself. At this time, the lithium bromide solution must be regenerated. When regenerating the lithium bromide solution, according to the changes in various indicators, various reagents are added to the sealed reactor, and the impurities are removed under high temperature and pressure conditions, so that the solution indicators meet the range specified in the industry standard HG/T2822—2012 "Lithium Bromide Solution for Refrigerators". After regeneration, the solution will have the same refrigeration effect and corrosion inhibition effect as a new solution. This regeneration method can only be carried out in the solution factory.

Basic Knowledge:
The Corrosiveness of the Lithium Bromide Solution to Metal Materials:
The lithium bromide solution is corrosive to metal materials, and oxygen is the main factor promoting corrosion. Therefore, in the lithium bromide absorption unit, isolating oxygen is the most fundamental anti-corrosion measure.
Dual Automatic Air Extraction System:
Factors Affecting the Corrosion of Metal Materials by the Lithium Bromide Solution:
Several factors affecting the corrosion of metal materials by the lithium bromide solution include the concentration of the solution, the temperature of the solution, and the alkalinity of the solution. Among them, the temperature of the solution has the greatest impact on the corrosion.

Temperature of the Solution:
When the temperature of the solution exceeds 180°C, the corrosion rate of the solution to metal materials increases sharply. Therefore, the temperature of the solution is not allowed to exceed 180°C. For steam-type units, there is an issue of steam superheat.
Regarding the Issue of Steam Superheat:
When the steam pressure is 0.4MPa, the corresponding saturated steam temperature is 152°C;
When the steam pressure is 0.6MPa, the corresponding saturated steam temperature is 165°C;
When the steam pressure is 0.8MPa, the corresponding saturated steam temperature is 175°C.
Operating Conditions of the Single-effect Lithium Bromide Absorption Refrigerator:
Steam pressure of 0.1～0.25 MPa or hot water at 75～140°C. The cycle thermal coefficient is 0.65～0.75.
Operating Conditions of the Double-effect Lithium Bromide Absorption Refrigerator:
Steam pressure ≥ 0.4 Mpa, and the cycle thermal coefficient ≥ 1.
Acidity and Alkalinity of the Solution
When PH≤7 or PH≥10.5, the corrosion of the solution to metal materials is aggravated. The optimal PH value is between 9 and 10.5.
In conclusion, the analysis and adjustment of the lithium bromide solution are of great importance. The usual analysis should include the acidity and alkalinity of the solution and the concentration of the corrosion inhibitor. Therefore, it is extremely irresponsible to add the solution to the unit without any analysis and adjustment.
What is a Corrosion Inhibitor?
As the name implies, a corrosion inhibitor refers to an additive that is added to the solution. Under the action of a chemical reaction, it can form a protective film on the metal surface to reduce or delay the corrosion of the solution to the steel plate.
There are usually two types: lithium chromate (Li₂CrO₄) and lithium molybdate (Li₂MoO₄). Generally, Li₂MoO₄ is selected as the corrosion inhibitor.
Corrosion Inhibition Mechanism
3Fe + 4H₂O + Li₂MoO₄ → Fe₃O₄ + MoO₂ + 2LiOH + 3H₂
From this, it can be seen that the corrosion inhibitor will be consumed after being added to the unit, and the acidity and alkalinity of the solution will also change. Therefore, in future service work, the solution needs to be analyzed and adjusted.