Introduction to Various Water Tanks in Air - conditioning Systems

2024-12-30

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 cold or heat energy, effectively solve the load fluctuation caused by the too small system and the frequent start-up problem of the main unit, so as to achieve the purpose of prolonging the equipment life and saving energy and electricity.

Expansion Tank

In fact, we have all seen the expansion tank in the project, which is composed of an airbag, 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 airbag, the system water will be squeezed into the airbag of the expansion tank under the action of the system pressure.
In this way, first, the nitrogen between the tank body and the airbag will be compressed, resulting in a decrease in its volume and an increase in pressure. Second, it will increase the accommodation space of the entire water in the system, reducing the system pressure until a new balance is reached between the system water pressure and the nitrogen pressure between the tank body and the airbag, and then the water inlet will stop.
When the system water pressure is less than the gas pressure in the expansion tank, the water in the airbag will be squeezed out under the pressure of the nitrogen between the tank body and the airbag and returned to the system. The water volume of the system decreases and the pressure rises, while the nitrogen volume between the tank body and the airbag increases and the pressure drops until a new balance is reached between the two, and the water stops being squeezed back into the system from the airbag. The pressure tank plays a role in adjusting the pressure fluctuation of the system.
Due to the regulating function of the airbag, the pressure tank is widely used in the control of small-range pressure fluctuations in the water system. When the pressure tank is applied to the hot water and heating systems, it is mainly used to eliminate the pressure fluctuations caused by the change of water temperature and avoid damaging other system control components.
We know that the expansion tank is also applied in the water supply system, which can eliminate the water hammer effect caused by the opening of the water valve and avoid the impact of the water hammer, so as to achieve the dynamic balance of the system. This is very common in the water supply system. Generally speaking, the maximum working pressure of the expansion tank is 8 to 10 kilograms, and the size ranges from 2 liters to 100 to 200 liters.
Water expands. Water has the greatest density at 4°C. When the temperature is less than 4°C or greater than 4°C, water will expand. When one ton of water is heated from 10°C to 90°C, its volume will probably increase by 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.

Energy Storage Tank

I understand that in order to achieve the purpose of energy conservation, in small central air-conditioning systems, increasing the water storage capacity of the buffer tank makes it an energy storage tank. Generally speaking, there is no essential difference among the energy storage tank, the expansion tank and the buffer tank.
The difference is only in the literal understanding. In actual application, they have similarities. The expansion tank is mainly used in small systems. In large systems, I think the buffer tank and the energy storage tank are exactly the same thing.

In addition, let's introduce more about the buffer tank:

Under the standard working conditions of air source heat pump heating, the start-stop times of the main unit (that is, the start times of the compressor) are important parameters to measure the service life of the main unit, just like the service life of household lamps is related to the on-off times. Adding a buffer tank is equivalent to increasing the energy storage of the system, making the system temperature change smoothly, naturally reducing the start-stop times of the main unit and prolonging its service life.
In some European and American countries, the buffer tank is a standard configuration. Some manufacturers' commissioning personnel even refuse to commission in the construction site of the equipment room without a buffer tank.
During long-term use, it has been found that in addition to the above advantages, the buffer tank also has some unique functions:

Forced automatic exhaust function in the system
If only the function of reducing the start-stop times of the main unit is required, the buffer tank can be installed in the water outlet pipeline of the main unit or the indoor return water pipeline. When the tank is in the system return water pipe, it will be found that the circulating water enters from the upper part of the tank and is discharged from the lower part. At this time, the gas in the water will accumulate in the upper space of the tank, and the pressure in the closed system will automatically and forcibly discharge the gas from the upper exhaust valve, without the phenomenon that when we install an exhaust valve on a small-diameter pipe, we need to drain a small section of water before discharging the gas, and a section of water will be blocked at the lower part, resulting in extremely poor exhaust effect.
Protection of the water pump, the power component in the water system
The bottom outlet of the buffer tank is connected to the water inlet of the water pump. The water entering this inlet is gas-free (the gas cannot come down from the upper part of the tank), and the cavitation phenomenon of the water pump impeller will be greatly reduced. The inhaled water is pure water, and the water circulated by the water pump will naturally push the gas in the system to the water outlet of the system, and then enter the upper water inlet of the buffer tank. The gas brought by the system water outlet will be forcibly excluded by the exhaust valve at the top of the tank, thus forming a virtuous cycle. In a system without a buffer tank, the water pump inhales gas-containing water, and the impeller will break the gas and send it into the system, resulting in the existence of gas everywhere in the system, making it more difficult to exhaust. This causes the flow switch of the water pump to often alarm, constantly start and stop, greatly increasing the commissioning difficulty.
Avoiding high-pressure protection of the main unit
The water entering the plate heat exchanger of the main unit is pure (gas-free) water, which can better exchange energy and increase the heat transfer efficiency.
Easier commissioning and faster terminal effect
When half of the water is replenished in the system, the commissioning can start.
The water flowing into the main unit and the water pump is pure and gas-free, and the heat will be quickly transferred to the terminal, greatly reducing the commissioning time. The author was engaged in air-conditioning construction in the early stage. When commissioning a villa, it could not be completed without three to four hours, because there was too much gas in the water, the flow switch often alarmed, and the main unit would also have high-pressure protection due to insufficient heat transfer. After the air coil was blocked by gas, the effect was not obvious. All these greatly prolonged the commissioning time. In the equipment room system with a buffer tank, the commissioning work can generally be completed in one to two hours.
More thorough system sewage discharge, preventing system blockage and more convenient sewage discharge
The impurities in the pipeline are constantly circulating. Because after passing through a vertical large-capacity tank, many impurities will accumulate at the bottom of the buffer tank, and the water quality passing through the Y-type filter will be much better, thus reducing the frequency of cleaning the filter. The sewage discharge port of the buffer tank is set at the bottom of the tank and equipped with a manual sewage valve, making cleaning more convenient.
More stable system operation in winter heating
When the small air-cooled heat pump is used in severe winter, the influence of defrosting time on indoor temperature is a headache problem. If an auxiliary electric heater is installed on the main unit to assist defrosting, the COP value of the main unit will be greatly reduced. After adding a buffer tank, when the main unit defrosts, the water stored in the tank at a certain temperature can meet the terminal circulation, completely avoiding the fluctuation of indoor temperature during the defrosting of the main unit.
Faster effect after startup
Because after the air conditioner is started for the first time, the previous cold or heat energy will be stored in the buffer tank. The insulation standard of the buffer tank is the same as that of the hot water tank (24 hours ± 3°C). Therefore, when it is used next time, the effect will naturally come faster. For example, turn off the air conditioner main unit one hour before getting off work, and only rely on the energy of the buffer tank. Through the circulation of the water pump, it can completely meet our needs until getting off work, and the effect is still excellent when going to work the next day.

Special Application Chapter:

For various reasons, we will also encounter some difficulties in the water system. For example, after the system is completed, it is found that the main unit seems to be selected too small to meet the heat demand at the terminal.
Suggestion: During the transformation, only need to 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 part of the energy through auxiliary electric heating to meet the terminal use.
If conditions permit, use a boiler, a flat-plate solar energy, or waste heat energy, etc. to supplement the energy of the buffer tank (with coils).
Use a small air-cooled heat pump main unit to connect with the increased circulation port of the buffer tank to supplement energy.
The above can also be implemented as the energy-saving scheme for the owner. The buffer tank needs special production. Just communicate with the salesperson in advance when ordering.

The following points need to be noted:

If the buffer tank is connected in series in the system, and the inlet and outlet diameters are unreasonable, it will cause excessive increase in system resistance.
Suggestion: For 40L to 200L buffer tanks, the inlet and outlet diameters are DN40; for 300L to 500L buffer tanks, the inlet and outlet interfaces are changed to double DN40 ports for inlet and double DN40 ports for outlet. Use double DN40 diameters to replace large diameters. Do not use DN50 (the tank is arc-shaped, the internal stainless steel is generally 1.2 - 1.5mm, the internal diameter of the tank is generally 470mm or 600mm, the weld with too large an interface will have hidden dangers during use, and the requirements for installation torque are also very high).
The position of the water inlet of the buffer tank should not be too low.
If the water inlet is too low, it will greatly affect the temperature at the bottom of the tank, which may lead to inaccurate induction of the return water by the main unit and cause unstable system operation. (Because the water sucked in by the return water port of the main unit is the water at the bottom of the buffer tank, and the water replenishment process will cause a large temperature change at the bottom of the tank.)
Suggestion: The water inlet of the buffer tank is near the middle of the tank.