Dec 11, 2016
It is an understatement to say that the centrifugal pump is one of the most important components of a heating, ventilation and air conditioning (HVAC) system. Boilers, chillers, domestic water systems, cooling towers and the like are so interdependent on the pump for the system to operate smoothly, that it makes correct pump selection an essential and vital component of the engineers’ design brief. But, despite the seemingly straightforward way in which a pump functions, they come in many different configurations.
In the broad spectrum of pumps, there are only two main types. Rotodynamic and Centrifugal. For HVAC systems, the centrifugal type is used so this article will confine itself to this type only. Within this type, there are a variety of configurations. Masterflow Solutions have a range of pumps in each configuration readily available. There are a few things you should know when it comes to selecting centrifugal pumps for HVAC applications.
There are various types of centrifugal pumps, including single and multistage, circulator, split case, and vertical in-line pumps. Each type is ideal for a specific situation. For instance, circulating pumps are ideal for low-pressure, low-capacity systems. Split case pumps are good for use in high-capacity systems and where there is an extremely high suction static pressure resting on the pump. Vertical in-line pumps are ideal for plantrooms where floor space is limited. In the absence of these special considerations, the close and long coupled BPO pumps are the preferred choice being suited to most systems. The table below summarises these various configurations and their common application.
It is important to choose a pump with the correct size of impeller and motor for the task that you require it for. People often add additional safety margins into their flow & friction calculations leading them to choose oversized pumps often in the belief that “big is beautiful” will solve their problem. However, an oversized pump will cause more problems than it was meant to solve especially when the pump operates at the extreme right of its curve or in some cases beyond its curve leading to noisy vibrations and premature bearing failure. When selecting a pump for peak flow rate conditions, the pump selection then must be checked when the system runs at less than peak.
The truth is that that most of the year these systems do not always work at peak performance. Hence the importance of carefully evaluating the pump selection at the less than peak conditions. The inclusion of Variable Speed Drives has helped immensely on this issue, but cannot be automatically considered a substitute for evaluation of each specific pump selection at various system loads.
Motor speed can be either 2 pole or 4 pole. ( 6 and 8 pole is also available in synchronous speed motors, but rarely used in HVAC system). A frequently asked question is “which is best – 2 or 4 pole?” The answer to this will often depend more on the duty point than anything else. As a rule of thumb, for flow rate up to 50l/sec, once the required head gets higher than 400kpa the 2 pole pump may be the only option. Experts recommend, however, that pumps at 1450 rpm are the ideal. This is because they are running half the speed of a 2 pole pump, resulting in less wear & tear, less noise and fewer bearing changes, which means lower maintenance costs of the same period of time as their faster 2 pole cousins. Definitely a plus for the end user!
Consider these factors when selecting centrifugal pumps on your HVAC system. Should you have any further question, you can contact us and we will gladly assist you.