HVAC Pumping: Pump Types
One decision engineers face is the type of pump to specify. The most common type of pumps found in hydronic heating and cooling applications are end-suction pumps.
They are relatively inexpensive and offer pumping efficiency at the rated condition, making them a practical choice for this application.
Inline vertical multistage pumps have been under-used in these applications. This is commonly used in more demanding applications such as boiler feed systems for steam generating plants and pressure boosting systems for multistory buildings. The modern version of this pump has been used since the late 1970s and has grown in size and versatility.
Vertical multistage pumps have flow capacities that reach over 1,000 gpm and efficiency reaching 80 percent and higher. There are many reasons to consider this style of pump for hydronic heating and cooling systems. A key benefit is the small footprint. On average, the floor space required for the inline multistage pump is 25 percent of the floor space required for alternative technologies.
These pumps can be prone to fewer problems because of the hydraulic design and use of smaller diameter impellers. Inline multistage pumps also have a wider AOR due to this hydraulic design. Because most of the loading on an inline multistage impeller is axial (parallel with the pump shaft), shaft deflection is typically not present.
Advancement in chiller designs and controls have resulted in more variable primary-based designs and a wide range of flow rates. Chillers have minimum flow requirements that often result in pumps operating at low flow, often below the minimum continuous stable flow.
Because of the wider AOR for multistage pumps, especially at low flow, this makes them a solution to variable primary-based designs. Using parallel connected pumps also allows for better low flow operation.
There are further design features that can make installation of inline multistage pumps easier. For instance, installation steps such as grouting and pump shaft to motor shaft alignment are not required. Due to the low inertia design and the lack of radial loading on the pump shaft, inline multistage pumps have low vibration levels, which can negate the need for vibration isolation. Another benefit to the inline multistage pumps is that the pump bearings are lubricated by the pumped liquid.
This results in elimination of a routine maintenance step (greasing pump bearings).
From a service and repair standpoint, inline multistage pumps are some of the easiest pumps to work on. Most have single piece cartridge mechanical seals that can be replaced without removing any part of the pump casing from the piping. Many of the larger sizes—typically 15 horsepower (hp) or 11 kilowatt (kW) and greater—have spacer couplings that allow for seal replacement without motor removal. In the event of a major repair, the entire rotating assembly is offered as a kit and can be replaced on-site while the bottom volute casing remains in place.
With these inline multistage pumps, end users can provide pump repair on-site and, in many cases, avoid a possibly costly field service call.
Advances in Pump & Motor Controls
Buildings account for nearly 40 percent of global energy consumption. There are opportunities for efficiency gains in the operation of pump systems and other HVAC equipment.
Using smarter pump controls and efficient motors is easier than ever and will not only reduce pump energy usage, but also provides greater efficiencies for larger HVAC systems.
The highest motor efficiency level recognized by the U.S. Department of Energy is NEMA Premium. The equivalent efficiency level to NEMA Premium in the European Union is IE3. The EU recognizes efficiency levels of IE4 and IE5. These efficiency levels have not yet made their way into the NEMA standard for motors in North America. However, motors in the IE4 and IE5 range are produced and sold worldwide, including in North America. These efficient, “above NEMA Premium” motors are typically of a permanent magnet design and are available in ratings that range up to 15 hp (11 kW). Using these motors on packaged pump systems can typically reduce energy consumption by 8 to 10 percent over NEMA Premium designs on a typical hydronic heating or cooling system.
One other area of improvement is advanced pump control logic, mainly originating from pump manufacturers. With the increased use of variable speed controls, this has resulted in pump manufacturers taking a harder look at how parallel-connected pumps are controlled.
Traditionally, the most common pump sequencing method is based on pump speed (when an operating pump reaches 95 percent of full speed, an additional pump is started, etc.). In today’s world, efficiency-based pump sequencing is possible given that pump curve information can be loaded into the controls and sensors can be used to continuously monitor efficiency.