Air conditioning chilled water pump lift estimation method (including examples)
The so-called selection and calculation of water pumps are actually estimates (many calculation formulas themselves are estimates), and more detailed and comprehensive considerations are accurate calculations.
Regarding the problem of excessive pump lift. The pump head selected by the design is too large, which will make the rich head exchange for an increase in flow rate, and the increase in flow rate will increase the pump noise. In particular, the increase in flow also increases the load of the water pump motor, the current increases, and the heat increases. "The bearing has been changed countless times" is still a trivial matter. It is very likely that the motor will be burned. If the pump lift is too small, the entire system will be paralyzed. The editor combined with previous estimation experience to cite examples so that readers can see at a glance.
Selection of HVAC pumps by simple estimation method of pump head: usually choose a centrifugal clean water pump with a specific speed of ns between 130 and 150. The flow of the pump should be 1.1 to 1.2 times the rated flow of the chiller (a single unit is 1.1, two units in parallel Take 1.2. According to estimation, the loss along the way per 100 meters of pipe length can be roughly taken as 5mH2O, and the pump head (mH2O):
Hmax=△P1+△P2+0.05L(1+K)
△P1 is the water pressure drop of the evaporator of the chiller.
△P2 is the water pressure drop of the one in the loop that accounts for the largest water pressure loss of the air conditioner's end device.
L is the tube length of the most unfavorable loop
K is the ratio of the sum of the equivalent lengths of local resistance in the most unfavorable loop to the total length of the straight pipe. When the most unfavorable loop is longer, the value of K is 0.2～ 0.3, and when the most unfavorable loop is short, the value of K is 0.4～0.6
Note: Here L is the tube length of the most unfavorable loop
The 250M side pipe length in the example below
Practical estimation method of chilled water pump head What we are talking about here is the resistance composition of the closed air conditioning chilled water system, because this system is the most commonly used system.
1. The resistance of the chiller: Provided by the unit manufacturer, generally 60~100kPa.
2. Pipeline resistance: including frictional resistance and local resistance. The frictional resistance per unit length, that is, the biscuit group, depends on the comparison of technology and economy. If the value is large, the pipe diameter is small and the initial investment is low, but the energy consumption of the pump operation is large; if the value is small, the opposite is true. In the current design, the biscuit group of the cold water pipeline should be controlled within the range of 150~200Pa/m. When the pipe diameter is larger, the value can be smaller.
3. Air conditioner end device resistance: the end device types include fan coil unit, combined air conditioner, etc. Their resistance is provided by the manufacturer after calculating the coil configuration according to the parameters of the air entering and exiting the air conditioning coil, the cooling capacity, and the water temperature difference. Many rated operating conditions can be found on the product samples. This resistance is generally in the range of 20~50kPa.
4. The resistance of the regulating valve: The air-conditioned room always requires the room temperature control. The installation of an electric two-way regulating valve on the waterway of the air-conditioning terminal device is a means to achieve room temperature control. The specification of the two-way valve is selected by the flow capacity and allowable pressure drop when the valve is fully open. If the allowable pressure drop is large, the control performance of the valve is good; if the value is small, the control performance is poor. The percentage of the pressure drop when the valve is fully open to the total pressure drop of the branch is called the valve authority. Water system design requires valve authority S>0.3, so the allowable pressure drop of the two-way regulating valve is generally not less than 40kPa.
Application example: Based on the above, the pressure loss of the air conditioning water system of a high-rise building about 60m high can be roughly estimated, that is, the required head of the circulating water pump:
1. Resistance of the chiller: 80kPa (8m water column);
Pipeline resistance: Take the resistance of the decontamination device, water trap, water separator and pipeline in the refrigerating machine room to 50kPa; take the pipeline length of the transmission and distribution side 250m and the specific friction resistance of 200Pa/m, then the friction resistance is 250 *200=50000Pa=50kPa; if considering that the local resistance on the transmission and distribution side is 50% of the friction resistance, the local resistance is 50kPa*0.5=25kPa; the total resistance of the system pipeline is 50kPa+50kPa+25kPa=125kPa (12.5m water column) );
3. The resistance of the air conditioner terminal device: the resistance of the combined air conditioner is generally greater than the resistance of the fan coil, so the resistance of the former is 45kPa (4.5 water column);
4. The resistance of the two-way regulating valve: take 40kPa (0.4 water column).
5. So, the sum of the resistance of each part of the water system is: 80kPa+125kPa+45kPa+40kPa=290kPa (29m water column)
6. Pump head: take 10% safety factor, then head H=29m*1.1=31.9m.
Note: The system used this time is a closed air conditioning cold water system, and the pump lift has nothing to do with the floor height. The height of the floor only affects the static pressure of the machinery and equipment.
