What is Flow Rate of The Pump / How to Calculate Pump Flow Rate / How to Choose the Suitable Pump for Flow Rate
In a centrifugal pump, Flow (Q) is the useful volume of liquid sent to the pump discharge nozzle in a period of time. Commonly used units for volume; m³/h, m³/s, l/s, l/h. Besides internal flows, losses due to leakage are not pay attention within the pump flow rate. The flow rate of the centrifugal pump is independent of the density of the pumped liquid. The viscosity of the pumped liquid affects the flow rate of the centrifugal pump. If the viscosity increases, the flow rate decreases, if the viscosity decreases, the flow rate increases. Flow rate changes in proportion to pump rotation speed.
The head (H) of a centrifugal pump is the useful mechanical energy that the pump gives to the fluid, measured between the inlet and outlet port of the pump. It is expressed in unit [m] (Nm of energy per unit weight of fluid). A centrifugal pump gives different liquids at the same rotational speed the same height regardless of their density. If the viscosity of the liquid changes, the resulting head also changes.
What is the minimum flow rate?
Protects a centrifugal pump against overheating and unstable flow rate (noise, vibration) The minimum flow rate in centrifugal pumps is 15% of the Qopt optimum flow.
How is the Pump Flow Rate and Pressure Regulate?
You can achieve variable speed manually with fixed multi-speed pumps or automatically using electronically controlled pumps. In addition to reducing energy consumption, speed-controlled pumps always operate at the optimum differential pressure. This minimizes noise in the piping system and increases living comfort.
For pumps where the speed is controlled manually, the flow rate is adjusted manually by selecting one of the fixed speeds. Flow rate will definitely not meet the need completely. In electronically controlled pumps, the speed is automatically adjusted to the speed needed to obtain the required flow rate (variable speed control). The pressure generated by the pump is constantly monitored and the pump speed is adjusted to deliver the required pressure. As the demand increases, the pressure starts to decrease and the speed of the pump is increased to compensate for this. As the demand decreases, the pressure increases and the speed of the pump is reduced to reach the correct pressure level.
There are different ways to adjust the pumps:
In constant pressure (Δpc) mode, the same pressure is given up to the highest speed regardless of the flow.
In linear variable pressure (pv) mode, preset pressure is given at the highest speed. The lower the velocity (and flow rate), the lower the pressure. This is to simulate the pump system curve. When the flow is low, the demand for pressure is also low.
The fully variable pressure (pv) curve is the same as the linear one; the only difference is that the pressure follows the quadratic curve (rather than the linear line) so that the energy consumption is further reduced and better meets the actual need.
If the pump runs only at half speed:
• Flow rate decreases by 50%
• Head reduced by 75%
• However, power consumption drops by 87.5%.
How to Calculate Pump Flow Rate
The flow rate calculation of the centrifugal pump is made with the following formula.
Q = (V x n x µ) / 1000
Q: Flow rate (liter / minute)
V: Delivery volume of the pump (cm3/rev)
n: The revolution of the electric motor used (rev/min)
µ: The volumetric efficiency of the pump (0,90 - 0,95 can be taken)
The following formula is used to calculate the flow rates of the Centrifugal Pump at different speeds.
Q2 = Q1*n2/n1
Flow rate changes in proportion to pump rotation speed.
How to Choose the Suitable Pump for Flow Rate
The variables of operating conditions that should be known in the selection of the pump type are the following: Flow rate (Q) and the desired head (H) at the operating point. The size and speed of the pump are determined from the pump selection curves. Other parameters of the selected pump such as efficiency, pump motor power and NPSH values are also read from the pump performance curves.
Pumps should be selected in accordance with the required flow rate and pipe resistance. When using speed-controlled pumps, always choose a pump where the duty point is as close to the best efficiency point as possible. Often there are several alternatives, and an important rule of thumb is to choose a pump that is within ± 10% of the best efficiency point. When using a speed-controlled pump, the duty point should always be within 10% of the best efficiency point. This will ensure that there is a sufficiently large flow area to be able to adjust while regulating the pump.
Do not choose the pumps too big! Heat exchange is almost the same, but pumps consume much more energy.
Failure of a pump in heating systems causes a decrease in indoor climate comfort. For this reason, it is recommended that in large buildings, instead of one pump, more pumps should be used to back up each other and to provide a certain level of comfort even when one pump fails. If one pump is capable of achieving the total flow, the other pump works as a backup. Or the total flow is handled by a few pumps, all of which only work fully efficiently when needed.