# Selection of calculation methods for circulating DC pumps

Published:2020-12-22

**1. Selection method of ****circulating DC pump**

The general method of **circulating DC pump** selection is to obtain the water flow required by the floor heating system and the head required to overcome the resistance of the floor heating system pipe network and the wall-hung boiler itself based on the results of hydraulic calculations. Comprehensive consideration of the **circulating DC pump** in the floor heating system For work efficiency, choose a suitable **circulating DC pump**.

**1.1 System flow calculation method:**

**Formula 1**

G=3.6Q/C(Tg-Th)

G—Required flow of heating pipe network, m3/h

Q—The heating load required by the house, kW

C—Specific heat of water, kJ/(kg•℃)

Tg—heating water temperature, K

Th—heating return water temperature, K

**Formula 2**

Q=K1K2qA

Q—The heat load required for heating of houses, kcal/h;

K1—Considering the safety factor of neighbor heating asynchronous, take 1.2 here;

K2—Considering the safety factor of intermittent heating, take 1.2 here;

q—The estimated value of standard residential heat index, kcal/m2;

A—standard residential building area, m2;

**1.2 System resistance**

System resistance is divided into pressure loss along the way, local pressure loss and internal resistance of the machine. Pressure loss along the way refers to the continuous and consistent pressure loss in the pipeline; local pressure loss refers to the special components in the pipeline system due to changes The direction of water flow, or the discontinuous pressure loss caused by the narrowing of local water channels (such as reduced diameters, tees, valves, connectors, filters, etc.); the internal resistance of the machine is the resistance of the machine itself.

**1.2.1 Pressure loss along the way**

The floor heating pipe is a round pipe and the inner wall is relatively smooth, which is of low roughness. Choose the pressure loss meter along the way.

**The calculation formula is as follows:**

**Formula 3**

Hf=λ•L/D•V2/2g,

Hf—pressure loss along the way, mm/m

λ—Friction resistance coefficient (not a fixed value)

L—Length of loop water pipe, m

D—pipe inner diameter, m

V—average water velocity m/s

Re<2300 is laminar flow:

**Formula 4**

λ=64/Re

Re>2300 is turbulent flow

**Formula 5**

λ=0.316Re-0.25

**Formula 6**

Is used to judge the water flow mode: laminar flow or turbulent flow

Table 2 Water temperature and fluid viscosity

Re=VD/γ

γ: dynamic viscosity coefficient, m2/s

**1.2.2 Local pressure loss**

The local pressure loss is mainly limited by the flow capacity of some valves and filters. The calculation formula for selection is as follows:

**Formula 7**

ΔP=102(G/KV0.01)2

ΔP; local pressure loss, mmh2o

G—Water flow required for heating pipe network, l/h

KV0.01—Flow capacity (pressure difference equal to 0.01bar), l/h

**1.2.3 The internal resistance of the machine itself is a measured value. **Because the wall-hung boiler industry has a relatively high start and the standardization is better, the internal resistance of the same type of products from different manufacturers is not much different.

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