Selection of Constant Pressure Water Supply Device for HVAC System

The function of the water replenishment device in the HVAC system is to ensure that the hot and cold medium (water) in the heating or central air-conditioning water system is not empty, vaporized, or overpressurized in the system, and maintains a certain pressure for the system to circulate , to ensure the stable and normal cooling and heat exchange of the system.

At present, the following types of constant pressure water supply devices are commonly used in HVAC systems:

①.Constant pressure replenishment device for expansion tank;

②.Constant pressure tank constant pressure water supply device;

③. Frequency conversion pump constant pressure water replenishment device; other devices such as continuous water replenishment pump water replenishment, water ejector water replenishment, tap water direct water replenishment, etc., are not introduced here because of their small scope of application or obvious defects.

1. Expansion Water Tank: The principle of constant pressure of the expansion tank: The principle of constant pressure of the expansion tank is to realize the function of replenishing water (overflow) through the buffer adjustment of the volume of the water tank and the control of the high and low water levels of the water tank, so as to adjust the water temperature change or leakage caused by the system. The volume change of the system medium (water) keeps the pressure of the cold and hot medium (water) of the system relatively constant. It is one of the constant pressure devices commonly used in small and medium-sized systems and air-conditioning water systems. The location of the expansion tank: The location of the expansion tank should be selected according to specific factors such as system type, radius of action, height of the building, and water supply temperature. The installation position and height are different, and the working conditions for the system are also different. For a reliable system, its operating conditions must meet the requirements of no vaporization, no overpressure, no emptying, and sufficient cycle power. The open-type expansion tank sets the water tank at the highest point of the system, usually connected to the return water main pipe of the suction port of the circulating water pump.

Classification of expansion tank types: separate (high) and closed (floor) closed expansion tank volume calculation: Vt=Vs(v2/v1-1-3αΔt)/(1-P1/P2)

Vt—volume of expansion tank: m3

Vs—total water capacity of the system: m3

v1—the specific volume of water at low temperature, m3/Kg;

v2—specific volume of water at high temperature, m3/Kg;

α—linear expansion coefficient, steel is 11.7×10-6°C-1, copper is 11.7×10-6°C-1

Δt—the maximum temperature difference in the water system, °C (generally 5)

P1—water pressure at low temperature, KpaP2—water pressure at high temperature, Kpa Determination of P1 and P2: P1, static pressure head of the tank + minimum pressure value P2 at the top of the system, maximum pressure during operation Calculation method of volume of the open expansion tank: Vp =αΔtVs

Vp---effective volume of expansion tank, m3

α---volume expansion coefficient of water, α=0.0006,1/℃

Δt---the maximum water temperature change in the system, ℃Vs---the total water capacity in the system, m3

Note: When the water tank is used for heating and cooling at the same time, it is calculated separately, and the larger value is taken. Features: (1) Advantages: It has simple installation, safety, less maintenance, low operating costs, stable pressure, no electricity, etc.; can effectively eliminate Overpressure of the system under abnormal working conditions. (2) Disadvantages: There is a space position requirement for the highest point; the system has oxidation and corrosion defects; it does not meet the needs of large areas and high-rise and super high-rise buildings.

2. Constant Pressure Tank: The working principle of the constant pressure tank: the constant pressure tank is a kind of constant pressure water supply device developed on the basis of the expansion tank, and its principle is the same as that of the closed expansion tank. When the system water temperature changes or the leakage causes the volume of water to change, the system pressure is stabilized within the preset pressure range due to the buffering effect of the high compressibility of the gas in the pressure tank. If the system pressure drops to the lower limit of the preset pressure, the electric contact relay will activate the supplementary water pump to supply water to the system until the pressure reaches the preset pressure upper limit. If the system pressure exceeds the set maximum pressure value, the safety valve will automatically release water to the soft water tank or the drainage system to reduce the pressure. To maintain the pressure balance of the system.

The device is composed of a pressure tank, a water supply pump, a safety valve, an electric contact pressure gauge, and a control box. The pressure of the constant pressure point in the system is determined: the pressure of the constant pressure point should consider two factors, one is that the system is not overpressured at any point when the system is running, and the other is that the system is not empty when the system is shut down. If the pressure at the constant pressure point is too high, the pressure at each point in the system will increase accordingly, causing pipelines, valves or equipment to operate under high pressure, resulting in strength damage or fatigue damage. If the pressure is set too low, the system will empty and become blocked, resulting in poor media circulation.

The working pressure value of the air pressure tank is determined by the following method (recommended) (1) The starting pressure of the water supply pump P1: P1=Po+0.005; the highest point pressure of the "Po" system (2) The stop pressure of the water supply pump P2: P2=(P1+0.1 )/β-0.1β: Working pressure ratio, generally 0.65~0.85 Volume: V=Vt/(1-β)Vt-adjusting water volume (m3), which is the flow rate of the supplementary water pump for 3 minutes, and keep the adjusted water level of the water tank not less than 200mm. Take half of the amount of expansion water when estimating.

3. Frequency Conversion Device: Basic principle: The frequency conversion speed regulation constant pressure water supply device is developed after the constant pressure tank, and is a combination of frequency conversion speed regulation technology and expansion tank technology. The basic principle is to adjust the frequency of the power supply through logic calculations according to the changes in the water pressure of the system collected by the sensor, and adjust the speed of the supplementary water pump smoothly and steplessly, that is, to adjust the supplementary water volume, so as to achieve the goal of relatively constant pressure at the constant pressure point of the system. The key equipment of this constant pressure method is the frequency converter. 

The key equipment of this constant pressure method is the frequency converter. Its working principle is to convert the general 50HZ alternating current into direct current, and then convert the direct current into the required frequency alternating current through the frequency converter. By changing the frequency of the water supply pump, the speed of the water supply pump and the amount of water supply can be adjusted to achieve the purpose of adjusting the water pressure of the system. The relationship between motor frequency and speed is: n=60f(1-S)/P or f=nP/60(1-S) In the formula: n is the speed of the water pump AC motor; f is the frequency of the power supply, Hz; S is the slip The rate is generally about 5%; P is the number of pole pairs of the motor. It can be seen from the above formula that when P and S are constant, the speed of the water pump motor is proportional to the frequency of the input power supply; from the characteristics of the water pump, the flow rate of the water pump is directly proportional to the speed, so adjusting the frequency of the power supply can directly adjust the flow rate of the supplementary water pump. Adjust the pressure change of the fluid medium in the system due to system temperature or leakage.

Water supply pump flow: The water supply pump flow rate (per hour) should not be less than 4% to 5% of the system water capacity. The frequency adjustment range of the frequency converter: the general adjustment range is between 5 and 50Hz, and the range of the frequency converter using a high-frequency power supply can reach 400Hz, but the requirements for the frequency converter itself and the motor are high, and it is not economical. In actual use, according to the specific conditions of the specific system, the system model should be established to calculate the relationship between the pressure of the system (sampling point) and the execution frequency of the replenishment pump, and adjust it during the debugging process, so as to finally realize that the water medium of the system will not be emptied or overpressured. And maintain a certain operating pressure purpose.

The specifications of the frequency converter are selected according to the parameters of the water supply pump: it is easier to achieve the above purpose with an industrial computer, and the closed-loop control of the water supply pump is carried out by using frequency conversion speed regulation technology and special industrial computer (PLC) technology. According to the two parameters of the instantaneous water loss in the circulating water system and the corresponding pressure value, after processing by the analog module of the industrial computer, the frequency conversion speed regulator is controlled to automatically adjust the speed of the water pump, so that the pressure of the water supply point of the circulating water system is constant at the system on the hydrostatic pressure line, the effect of small pressure fluctuation and more energy saving can be achieved. Features: (1) Advantages: It has the advantages of constant pressure tanks, but compared with constant pressure tanks, it solves the problems of frequent start-up of the water supply pump, affecting life, and consuming more power; the volume of the tank body is small and the space is small; the operation is more convenient and more humane; Adapt to large-area high-rise HVAC systems. (2) Disadvantages: The equipment is expensive and the investment is large, and the frequency and system constant pressure models need to be established for each individual, which requires high technical requirements for the use, adjustment, and maintenance personnel; compared with the energy consumption of the expansion tank, it is affected by the power supply.

The selection of any device, including the HVAC constant pressure water supply device, must objectively analyze the specific situation and needs. Any so-called scientifically advanced device also has its scope and limitations. It should proceed from reality and comprehensively evaluate and select from various aspects such as safety, reliability, stability, advancement, economy, and operability. The most expensive or the so-called most advanced is not necessarily the most suitable, the most suitable is the best and most scientific.