Air Conditioning System: Different Water Tanks

2024-08-27

Buffer tank
The buffer tank is actually connected in series in the central air conditioning system to increase the water capacity of the small system, store cooling capacity or heat, effectively solve the load fluctuation and frequent start-up problems of the host caused by the small system, so as to achieve the purposes of prolonging equipment life and saving energy and electricity.

Expansion tank
In fact, everyone has seen the expansion tank in the project. It is composed of an air bag, a carbon steel tank body and flanges. When the system water pressure is greater than the nitrogen pressure between the carbon steel tank body of the expansion tank and the air bag, the system water will be squeezed into the expansion tank air bag under the action of the system pressure.
This will, on the one hand, compress the nitrogen between the tank body and the air bag, reducing its volume and increasing its pressure. On the other hand, it will increase the accommodation space of the entire water in the system and reduce the system pressure. The water inlet will stop until the system water pressure and the nitrogen pressure between the tank body and the air bag reach a new balance.
When the system water pressure is less than the gas pressure in the expansion tank, the water in the air bag will be extruded back to the system under the pressure of the nitrogen between the tank body and the air bag. The system water volume decreases and the pressure rises. The volume of nitrogen between the tank body and the air bag increases and the pressure drops until the two reach a new balance. The water stops being squeezed back from the air bag to the system. The pressure tank plays a role in regulating system pressure fluctuations.
Due to the regulating effect of the air bag, the pressure tank is widely used in the control of small-scale pressure fluctuations in water systems. The pressure tank is used in hot water and heating systems mainly to eliminate pressure fluctuations caused by changes in water temperature and avoid damage to other system control elements.
We know that expansion tanks are also used in water supply systems. They can eliminate the water hammer effect caused by the opening of water valves and avoid the impact of water hammers, thus achieving dynamic balance of the system. This is very common in water supply systems. Generally speaking, the maximum working pressure of an expansion tank is eight to ten kilograms, and the size ranges from two liters to one or two hundred liters.
Water expands. Water has the highest density at 4°C. When the temperature is less than 4°C and greater than 4°C, water will expand. When one ton of water is heated from 10°C to 90°C, the volume will increase by about 36.5 liters. If there is no expansion tank, the water will damage other connecting parts. Therefore, the role of the expansion tank cannot be ignored, especially in small systems.

Air Conditioning System: Different Water Tanks1724762073988

Energy storage tank
In my understanding, in order to achieve the purpose of energy saving, in small central air conditioners, increasing the water storage capacity of the buffer tank becomes an energy storage tank. Generally speaking, there is no essential difference among the energy storage tank, expansion tank and buffer tank.
The difference is only in literal understanding. In practical applications, they have the same functions in some respects. The expansion tank is mainly used in small systems. In our large systems, I think the buffer tank and energy storage tank are exactly the same thing.
In addition, let's introduce some more content about the buffer tank:
Under the standard working conditions of air source heat pump heating, the start-stop times of the host (that is, the number of compressor starts) is an important parameter to measure the service life of the host. Similar to the service life of household lamp switches being related to the number of on and off times, adding a buffer tank is equivalent to an increase in system energy storage, stable system temperature changes, and naturally a reduction in the start-stop times of the host, thus prolonging the service life.
In some European and American countries, the buffer tank is a standard configuration. Some factory debuggers even refuse to debug on construction sites without buffer tanks.
During long-term use, it is found that in addition to the above advantages, the buffer tank has also extended some unique functions:

Compulsory automatic exhaust function in the system
If only the function of reducing the start-stop times of the host is required, the buffer tank can be installed in the host outlet water pipeline or the indoor return water pipeline. When the tank is in the system return water pipeline, it will be found that the circulating water enters from the upper part of the tank and discharges from the lower part. At this time, the gas in the water will accumulate in the upper space of the tank. The pressure in the closed system will automatically and forcibly discharge the gas from the upper exhaust valve. There will be no phenomenon that we install an exhaust valve on a small-diameter pipe and need to drain a small section of water before discharging the gas, and then a section of water body will be blocked at the lower part, resulting in extremely poor exhaust effect.
Protection of the pump, the power component in the water system
The bottom outlet of the buffer tank is connected to the suction port of the pump. The water entering from this suction port is free of gas (the gas cannot come down in the upper part of the tank). The cavitation phenomenon of the pump impeller will be greatly reduced. The inhaled water is pure water. The water circulated by the pump will naturally push the gas in the system to the system outlet, and then enter the upper water inlet of the buffer tank. The gas brought by the system outlet water will be forcibly discharged by the exhaust valve at the top of the tank, thus forming a virtuous cycle. In a system without a buffer tank, the pump sucks in water containing gas. The impeller will break the gas and input it into the system, causing gas to exist everywhere in the system, making it more difficult to exhaust. This causes the flow switch of the pump to frequently alarm and start and stop continuously, greatly increasing the difficulty of debugging.
Avoiding high-pressure protection of the host
The water entering the main engine plate heat exchanger is pure (free of gas) water, which can better exchange energy and increase heat exchange efficiency.
Easier debugging and quicker effect at the end
When half of the system is filled with water, debugging can begin. The water flowing into the host and the pump is pure and free of gas. The heat will be quickly transferred to the end, greatly reducing the debugging time. In the early days, the author was engaged in air conditioning construction. When debugging a villa, it could not be completed without three to four hours because there was too much gas in the water. The flow switch frequently alarmed. Due to insufficient heat exchange of the host, high-pressure protection would also occur. After air duct blockage, the effect was not obvious. All these factors greatly extended the debugging time. In a machine room system with a buffer tank, the debugging work can generally be completed in one to two hours.
More thorough system sewage discharge, preventing system blockage and more convenient sewage discharge
Impurities in the pipeline will accumulate at the bottom of the buffer tank after continuously circulating through a large vertical water tank. The water quality passing through the Y-type filter will be much better, thereby reducing the frequency of cleaning the filter. The sewage outlet of the buffer tank is set at the bottom of the tank and is equipped with a manual sewage valve for more convenient cleaning.
For winter heating, the system operates more stably
When small air-cooled heat pumps are used in severe winters, the defrosting time is a headache problem for the indoor temperature. If auxiliary electric heating is provided in the host to assist defrosting, the COP value of the host will be greatly reduced. After adding a buffer tank, when the host defrosts, the water stored in the tank at a certain temperature can meet the end circulation, completely avoiding the fluctuation of indoor temperature during host defrosting.
After startup, the effect comes faster
Because after the air conditioner is turned on for the first time, the previous cooling capacity or heat will be stored in the buffer tank.
The insulation standard of the buffer tank is the same as that of the hot water tank (±3°C in 24 hours). Therefore, when it is used next time, the effect will naturally come faster. For example in life: turn off the air conditioning host one hour before getting off work. Relying only on the energy of the buffer tank and circulating through the water pump can fully meet our needs until we get off work and go to work the next day. The effect is still excellent.

Special application chapter:
Due to various reasons, we will also encounter some difficulties in water systems. For example, after the system is completed, it is found that the host seems to be undersized and cannot meet the heat usage at the end.
Suggestion: When renovating, just connect a special buffer tank in series in the return water pipeline.

Add electric heating to the buffer tank. When the energy is insufficient, supplement a part of the energy through auxiliary electric heating to meet the end use.
If conditions permit, use a boiler, a flat-plate solar energy collector, or waste heat energy to supplement energy to the buffer tank (with coil).
Use a small air-cooled heat pump host and connect it to the increased circulation port of the buffer tank to supplement energy.
The above can also be implemented as an energy-saving scheme for the owner. The buffer tank needs to be specially produced. Just communicate with the salesperson in advance when placing an order.

The following points need attention:

If the water tank is connected in series in the system and the inlet and return water diameters are unreasonably sized, it will cause excessive increase in system resistance.
Suggestion: For buffer tanks ranging from 40 liters to 200 liters, the inlet and return water diameters are DN40; for buffer tanks ranging from 300 liters to 500 liters, the inlet and return water connections are changed to double DN40 inlets and double DN40 outlets. Use double DN40 diameters instead of large diameters. Never make DN50 (the water tank is circular arc-shaped. The internal stainless steel is generally 1.2-1.5mm. The internal diameter of the water tank is generally 470mm or 600mm. The weld at an excessively large interface will have hidden dangers during use and also has very high requirements for installation torque).
The location of the water replenishment port of the buffer tank should not be too low.
A too low water replenishment port will greatly affect the temperature at the bottom of the tank and may cause inaccurate sensing of the return water by the host, resulting in unstable system operation. (Because the water sucked in by the return water port of the host is the water at the bottom of the buffer tank. The water replenishment process will cause a large temperature change at the bottom of the tank.)
Suggestion: The water replenishment port of the buffer tank is near the middle of the tank.
The selection of the buffer tank should be appropriate.