Another fact to consider when it comes to pumps is the design point. It is the point at which a pump operates at maximum efficiency. It is determined by the flow rate and the total head at the respective pump speed. Nair asks: “What are the conditions of these pumps in actual operations? Because only 1% of the total 365 days, the pump will run at the design point.”
He explains: “We are designing a pump at a certain capacity, and at that design capacity, the pump will not run for 365 days, because this is the maximum demand. So the demand will fluctuate. For example, during winter, the demand is less, the pump will run at a low speed. But what will be the efficiency level when it runs at low flow or low demand? This is what people should look for.”
District cooling plants are very keen on such numbers, Nair says, adding: “They look at all specific points at different levels. But there are not many district cooling plants, only three or four. However, there are hundreds of buildings and shopping malls, residential buildings and hospitality buildings and all of them use chilled water pumps because there they have standalone chillers.”
According to the HVAC pump design objectives, a modern design will utilise computational fluid dynamics design tools available to improve the efficiency in three aspects of the pumps.
One is higher efficiency levels, second is wider efficiency range for flow rate perspective at constant diameters, and third is sustained efficiency levels as impeller diameters are reduced.
In this context, Nair says, the pump manufacturers have come out with a selection criteria called part load efficiency value (PLEV). “PLEV is a calculation that represents the efficiency of the pump at partial flow rates, and can be used to measure true pump performance within a hydraulic system. When you have this selection criteria, which was not there before, you can measure the pump efficiency at different percentages of operation. This is a good tool for the designers and also for the end users to measure the efficiency levels.
“Sometimes people use oversized pumps to meet the various demand fluctuations leading to inefficiencies. A pump is considered oversized when it does not operate at about 20% of the best efficiency point. So if it is plus or minus 20%, we say that the pump is not operating at its best efficiency point.”
Thinking outside the specification
Finally, Nair says that several consultants ask him to provide them with a list of pumps that can be used with a specific set of pump capacities. Although such a list can be provided, it will be generic, he says.
Nair explains: “There are some technical and engineering reasons why we are using certain types of split case pumps. If you go for larger capacities, you will use horizontal split case pumps, and there are certain engineering reasons behind this. The type of pumps has to be selected based on the hydraulic, your application, materials, etc.”
However, Nair says: “When the designer says he/she needs to use an end suction pump because of what it says in the specification, but the pump characteristics might be better with a multistage pump, then the designer or consultant who is reviewing the pump proposal should have the courage or openness to accept that a multistage pump could be better.”
Therefore, it is very important to think different and outside the range of specifications provided by the manufacturer in order to choose the right pump.