Selecting an HVAC pump

The flow rate, pressure losses, best efficiency point compared to the duty point, and the operating cost are all important factors when selecting a pump for HVAC applications.

05/21/2015


Learning Objectives: 

  • Understand the various pumps available for HVAC systems.
  • Learn about pump system configurations.
  • Know how to calculate HVAC loads in order to select the correct pump.

There are several types of pumps used in fluid transfer, but the most common used in today's HVAC systems is the centrifugal pump. Types of centrifugal pumps include volute or axial flow pumps. The volute takes water from the impeller and discharges the water perpendicular to the shaft. A centrifugal pump with a diffuser casing (axial flow pump) discharges the water parallel to the pump shaft.

Figure 1: The image illustrates a circulating pump installed in a vertical piping configuration. Courtesy: Armstrong Pumps

Centrifugal pumps are available in many types including circulator, single- and multi-stage end-suction, single- and multi-stage split-case, and vertical in-line pumps.

Circulating pumps are typically used in low pressure, low-capacity systems. The size of this system is typically under 150 gpm and not rated for more than 125 psig operating pressure. This type of pump is typically mounted directly in and supported by the piping system and is available with the motor in either the vertical or horizontal position. See Figure 1 for a standard circulating pump.

End-suction pumps are single-suction and can either be close- or flexible-coupled. A close-coupled pump has the impeller directly mounted to the motor shaft. A flexible-coupled end-suction pump has the impeller and the motor shaft separated by a flexible coupling. The benefit of using a close-coupled pump is that alignment of the motor shaft to the impeller is fixed. A flexible-coupled pump can become misaligned during maintenance. This can create issues if not properly reassembled by trained personnel. End-suction pumps are designed such that the incoming water enters the pump through the end in a horizontal manner. The water then changes direction and is discharged vertically, perpendicular to the suction. These pumps are typically installed on a solid base on the floor. An end-suction pump is capable of being used in HVAC systems with capacities up to 4000 gpm and 150 ft of head.

Figure 2: The figure illustrates a flexible-coupled end suction pump. The suction is the opening on the end of the pump whereas the discharge is vertical exiting the pump upward. Courtesy: Armstrong PumpsThe advantage to using a close-coupled pump is that it requires less floor space within a plant room for installation. One of the disadvantages of using a close-coupled pump in an HVAC system is the motor type. The motor is typically specially matched to the type of shaft and the seals for the pump. Flexible coupled pumps typically use standard motors. See Figure 2 for a typical flexible coupled end-suction pump.

Split-case pumps are similar to end-suction pumps in that they are flexible coupled between the motor and the pump. The assembly, including the motor and pump, is rigidly mounted to a common base-plate. Pump suction and discharge are arranged in the horizontal direction and are perpendicular to the shaft.

Split-case pumps are available either in single- or double-suction. To be a single-suction pump, the water enters the impeller from only one side. For double-suction, the fluid enters the impeller from both sides. Using double-suction reduces the risk of hydraulic imbalance. The reduction of hydraulic imbalance is one of the reasons why double-suction split-case pumps are preferred over single-suction.

Figure 3: This illustrates a horizontal split-case pump. The piping suction and discharge are horizontal to the floor while being perpendicular to the shaft. Courtesy: Armstrong Pumps

Split-case also may have multiple impellers for multi-stage operation. Multiple impellers provide increased available head within a single pump.

Split-case pumps are available as horizontal or vertical split-case. For horizontal split-case pumps, the impeller casing is split in the horizontal plane. For vertical split-case pumps, the impeller casing is split in the vertical plane. To have the casing split allows full access to the impeller for maintenance.

Split-case pumps are used mostly in fire protection systems but also have been used in the HVAC industry for large capacity systems. Their capacity range is up to 6500 gpm and 600 ft of head. These pumps are also available with increased operating pressures up to 400 psig. See Figure 3 for a typical horizontal split-case pump.

Figure 4: This image shows a vertical in-line pump. The flow of water will enter the pump on the right and discharge on the left. Courtesy: Armstrong Pumps

These pumps typically have a reduced footprint within a plant space and do not require inertia bases. Inertia bases are generally installed to reduce vibration from the rotating parts within the pump. discharge piping are arranged in the horizontal plane. Vertical in-line pumps are available as single- or double-suction.Vertical in-line pumps are close-coupled. The pump and motor are directly mounted on the pump casing. The pump is typically mounted and supported by the piping system in which it is installed. For larger capacity vertical in-line pumps, the pump assembly may be provided with a base for floor mounting. The pump suction and

Vertical in-line pumps have a capacity of up to 25,000 gpm and 300 ft of head. See Figure 4 for a typical vertical in-line pump.


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