Pumps are widely used in commercial buildings, manufacturing plants, and all different types of facilities; such as health care, schools, and municipalities. In most cases in the commercial sector, pumps are used in the heating, ventilation, and air condition (HVAC) system to supply heating or cooling to an area. Pumps are also used to supply domestic hot/cold water to the building and transfer waste water outwards.
Pumps are very important to the daily operation of different facilities. Since the pumps must be large enough to meet the system needs in the worst scenario, it is always oversized in capacity and therefore, costlier. This practice also creates a higher than necessary operating and maintenance cost. It also impacts other system components such as pipes and valves.
Most systems are designed with multiple pumps. Having many could not only reduce the pump size, but make the system more reliable with flexibility and redundancy. Usually, multiple pumps of the same size are operated in parallel. Adding an additional pump into operation will add flow and impact the head pressure.
In the HVAC industry, pump capacity is designed to counter the coldest winters and the hottest summers. As such, most of the time, the system is not running at full capacity. Variable speed drive (VFD) is commonly used now to adjust the speed to meet the partial load at different times. VFD changes the pump by changing the frequency of the power applied to the motor. Variable pump speeds are better as the system will have less pressure in partial load, which increases equipment lifespan. Furthermore, it will significantly save energy. According to the motor affinity law, the cube of the speed is directly proportional with the power requirement of the motor. Even a small change in motor speed will greatly impact the power needed. Since reducing the pump speed could significantly save energy, it is recommended to run more pumps at low speed instead of run one pump at high speed.
Usually, the pumps are on duty/standby mode. One pump will run continuously with varied speeds to meet the system load, while the other is on standby. However, if the pump on duty fails, the standby one will start. Our bottom line is to make sure the load is satisfied and the system operates properly.
In one of our sites, we have installed VFDs for each pump in a reheat system. The system could modulate the pump speed to maintain the differential pressure on terminal side. We also installed a flow meter and a power meter. The next step is to test the efficiency of the system in different modes and discover the best operation mode.