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Selection and Price of Heating Circulating Water Pumps

03 Apr 2023Email

The heating circulating water pump is the heart of the heating system, responsible for driving the heat medium to transfer heat energy. The selection and matching of the equipment directly affects the transmission effect and energy consumption. To achieve energy-efficient operation of the heating system and reduce heating costs, the analysis, discussion, and improvement of how to select and configure circulating water pumps are necessary.

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1Traditional selection principles and problems

The traditional selection of heating circulating water pumps usually involves several pumps in parallel to meet the flow and head requirements of the boiler room, heat network, and heat users, which can be called a single-stage circulation pump system. The flow rate is determined based on the maximum flow rate calculated by the heat load multiplied by 1.05. The head is selected based on the sum of the pressure loss of the heat source, heat network, and the most unfavorable loop under the determined flow rate, plus 2-3mH2O of surplus head. The number of pumps is determined based on the heating scale, generally selecting three pumps, operating two, and reserving one.

However, the circulation water pumps designed and configured based on the above principles have the following problems:

(1) Since the maximum circulating water flow calculated based on the heat load (heating area) is inconsistent with the total circulating water flow calculated based on the boiler's rated flow rate, the maximum circulating water flow calculated based on the heat load is usually much higher than the total circulating water flow calculated based on the boiler's rated flow rate. If not taken care of, it will cause the boiler to operate beyond its rated flow rate. Since the water resistance of the boiler is proportional to the square of the flow rate, it will significantly increase the pressure loss in the boiler room, and if a high-temperature water boiler operates as a low-temperature water boiler, the pressure loss will be even greater. The pressure loss of some boiler rooms can reach more than 0.3MPa, forcing the need to increase the pump's head and power, resulting in serious waste of electrical energy. Experienced designers or managers usually use a bypass pipe parallel to the boiler to ensure that the circulating water flowing through the boiler is at the rated flow rate. Although this measure can reduce some pump energy consumption, it does not fundamentally solve the problem.

(2) From an energy-saving perspective, the one-time water provided by the heating boiler in the intermittent supply system should be variable flow rate, and the quality should be adjusted. However, the traditional circulation water pump system designed based on the conventional principle is difficult to achieve variable flow rate regulation because it must ensure that the circulating water flowing through the boiler is not lower than the rated flow rate. When building heating systems that use household heat metering, when heat users have the ability to adjust actively, the circulation water pump should also be variable flow rate. Based on the above reasons, the traditional circulation water pump system design concept cannot meet the requirements of user-initiated adjustment.

(3) For some heating circulating water pump systems in boiler rooms, due to the design concept, the boiler operates beyond its rated flow rate, not only significantly increasing water resistance and causing electrical energy waste, but also causing the internal circulating water flow rate of the boiler to be too fast, resulting in low water wall temperature and low furnace temperature, leading to poor combustion conditions and low efficiency. The same problem exists when the primary heating network water operates at low temperatures.

 

In summary, it is necessary to explore and improve the selection and configuration of heating circulating water pumps to achieve energy-efficient operation of the heating system and reduce heating costs.

 

2Design concept of the secondary heating circulating water pump system

Based on the actual conditions, the traditional design concept has been broken to transform the single-stage circulating pump system that only satisfies the flow and lift requirements of the boiler room, heat network, and heat users into a two-stage circulating pump system. This involves setting up a circulating pump for the boiler room and another one for the heat network, as a transitional scheme to achieve the optimal design for distributed variable-frequency circulating water pumps.

 

(1) The flow rate of the heating circulating water pump in the boiler room is selected according to the rated flow rate of the boiler, and its lift is determined by adding the water resistance of the boiler at the rated flow rate and the resistance of the pipeline and accessories in the boiler room. There is no need to increase the excess pressure head. It can be one pump for one boiler, or two or more boilers for one pump, depending on the development of heating load and the regulation mode of heating operation, and run at fixed-frequency and constant flow rate. The heating circulating water pump in the boiler room only transports the rated circulating water volume required for boiler operation, and its lift only overcomes the water resistance of the boiler and the resistance of the pipeline and accessories in the boiler room at the rated flow rate. The flow rate and lift do not consider the excess quantity.

 

(2) The flow rate of the heating circulating water pump in the heat network is determined based on the maximum flow rate calculated for the system heating load, and its lift is selected by adding 2-3 mH2O excess pressure head to the resistance of the heat network and users at the maximum flow rate. To meet the demand for active regulation by heat users who need variable flow rate for once-through water supply system and individual metering for direct supply system, variable-frequency and variable-flow rate operation is adopted. The number of water pumps is also determined according to the development of heating load and operation regulation mode. The capacity can be matched with different sizes, and single pump is preferred.

 

(3) The inlet of the heating circulating water pump in the boiler room and that in the heat network are connected through a balanced pressure pipe, which has the same diameter as the adjacent pipeline. When the flow rate of the heat network circulating pump is greater than that of the boiler room circulating pump, part of the heat network return water flows to the inlet of the boiler room circulating pump through the balanced pressure pipe, and another part of it flows to the inlet of the heat network circulating pump and mixes with the boiler water supply. When the flow rate of the heat network circulating pump is less than that of the boiler room circulating pump, all the heat network return water flows to the inlet of the boiler room circulating pump after mixing with the boiler water supply in the balanced pressure pipe. It can be seen that by changing the direction of water flow in the balanced pressure pipe for different operating conditions, the coordination and balance of different circulating flow rates of the two-stage circulating pump can be automatically achieved.

 

3Design concept of the secondary heating circulating water pump system

Based on the actual conditions, the traditional design concept has been broken to transform the single-stage circulating pump system that only satisfies the flow and lift requirements of the boiler room, heat network, and heat users into a two-stage circulating pump system. This involves setting up a circulating pump for the boiler room and another one for the heat network, as a transitional scheme to achieve the optimal design for distributed variable-frequency circulating water pumps.

(1) The flow rate of the heating circulating water pump in the boiler room is selected according to the rated flow rate of the boiler, and its lift is determined by adding the water resistance of the boiler at the rated flow rate and the resistance of the pipeline and accessories in the boiler room. There is no need to increase the excess pressure head. It can be one pump for one boiler, or two or more boilers for one pump, depending on the development of heating load and the regulation mode of heating operation, and run at fixed-frequency and constant flow rate. The heating circulating water pump in the boiler room only transports the rated circulating water volume required for boiler operation, and its lift only overcomes the water resistance of the boiler and the resistance of the pipeline and accessories in the boiler room at the rated flow rate. The flow rate and lift do not consider the excess quantity.

(2) The flow rate of the heating circulating water pump in the heat network is determined based on the maximum flow rate calculated for the system heating load, and its lift is selected by adding 2-3 mH2O excess pressure head to the resistance of the heat network and users at the maximum flow rate. To meet the demand for active regulation by heat users who need variable flow rate for once-through water supply system and individual metering for direct supply system, variable-frequency and variable-flow rate operation is adopted. The number of water pumps is also determined according to the development of heating load and operation regulation mode. The capacity can be matched with different sizes, and single pump is preferred.

(3) The inlet of the heating circulating water pump in the boiler room and that in the heat network are connected through a balanced pressure pipe, which has the same diameter as the adjacent pipeline. When the flow rate of the heat network circulating pump is greater than that of the boiler room circulating pump, part of the heat network return water flows to the inlet of the boiler room circulating pump through the balanced pressure pipe, and another part of it flows to the inlet of the heat network circulating pump and mixes with the boiler water supply. When the flow rate of the heat network circulating pump is less than that of the boiler room circulating pump, all the heat network return water flows to the inlet of the boiler room circulating pump after mixing with the boiler water supply in the balanced pressure pipe. It can be seen that by changing the direction of water flow in the balanced pressure pipe for different operating conditions, the coordination and balance of different circulating flow rates of the two-stage circulating pump can be automatically achieved.

(4) For the hydraulic balance between the primary water of the heat exchange station in the secondary supply system, in addition to using electric valves installed at the inlet and outlet of the heat exchange station to control and adjust themselves, for small-scale secondary supply systems, to reduce investment, flow-limiting and resistance-limiting valves can also be installed at the inlet and outlet of the primary water of the heat exchange station for hydraulic balance adjustment between heat exchange stations. The method is to set the maximum flow rate of the flow-limiting and resistance-limiting valves according to the maximum heat load of each heat exchange station and the heating parameters of the primary water, and then fix the valve core by simple adjustment to ensure that the resistance characteristic coefficient remains unchanged.

According to this principle, a small-scale district heating system can implement automatic control only on the hot water circulating pump, and the primary water flow rate between each heat exchange station can be proportionally distributed based on the above principle to meet the heat exchange demand of the secondary network with changes in atmospheric temperature. This can eliminate the investment in electric valves and control equipment for the secondary network, reducing costs. In summary, the single-stage circulating pump of the heating circulating water pump in the heating boiler room is replaced with a two-stage circulating pump. Although significant energy-saving effects can be achieved and the working environment can be improved based on this concept, due to limitations in the development of the heat load, all equipment has not yet been fully installed according to the design scale. This still requires further exploration and refinement of design concepts and adjustment of scheduling and operation modes in practice.

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