Constant Water Flow System with Thermal Water Flow Sensor and

Variable Frequency Pump

Xiang Zheng Tu

 

It has been reported that electrical energy consumed by pumps, fans and compressors represents a significant proportion of the electricity used around the world. It is estimated that in industrial processes and building utilities, 72 % of electricity is consumed by motors, of which 63 % is used to drive fluid flow in pumps, fans and compressors.

Many heating, cooling and ventilation distribution systems operate at a constant flow rate, even though peak demand may only be required for a few hours. The conventional response to meeting the changing demand for heating and cooling within a building is to restrict flow to individual rooms, while maintaining peak flow in the central system. However, through the use of this approach, considerable energy is used and equipment lifetime is shortened.

For saving energy a better approach is to use a variable speed drive on pumps and fans to vary air or water flow to meet more precisely changing load demands. As shown in the above figure, a constant water flow system comprises a variable frequency pump and a POSIFA’s thermal flow sensor. When the water flow decreases in the user pipe the thermal flow sensor measurement signal decreases. This signal feedback to the variable frequency inverter and results its output frequency decreased. With decreased frequency the operation speed of the variable frequency pump is also decreased and the water flow in the user pipe starts to drop back as to original balance. When the water flow increases the similar process will happen only the direction is opposite.

There is an everyday analogy that can help explain the efficiency advantage of a variable frequency pump.  Imagine you are driving a car. If you are driving on a highway and entering a population area, speed must be reduced so that you do not risk your own and other lives. The best possible way to do that is to reduce motor-rotation speed by taking your foot off the gas pedal and, if necessary, changing to a lower gear. Another possibility would be to use the same gear, keeping your foot on the gas, and at the same time reducing speed simply by braking. This would not only cause wear in the engine and brakes, but also use a lot of fuel and reduce your overall control of the vehicle, which is the case for a "control valve."

In most traditional cases, the variable frequency pumps are controlled to maintain a constant pressure within air ducts or water pipes in which the pressures are measured by pressure differential flow sensors. The differential pressure flow sensor is based on Bernoulli’s Equation, where the pressure drop is a squared function of the fluid velocity.

This relationship can limit the ability of differential pressure sensors to measure large flow ranges. Generally the flow measurement range of 10-100 flow units (10:1 flow turndown) would require a differential pressure flow sensor range of 1-100 differential pressure units (100:1 differential pressure turndown). Therefore, the actual 10:1 flow turndown requires a 100:1 differential pressure flow transmitter turndown.

It would be much better to use the thermal flow sensors replacing the pressure differential flow sensors for controlling the variable frequency pumps. The thermal flow sensor measurement is based on heat transfer from a heated element. The measurement is in mass flow, and additional pressure and temperature correction is not required which is not like the pressure differential flow sensors. They also provide excellent accuracy and repeatability and are easy to install.