Selecting the right pump for a commercial HVAC system
Choice of pumping systems rarely is as straightforward as some would claim when designing an HVAC system for commercial building applications. To make an informed decision that considers operating expenses, energy costs, system reliability and longevity, contractors and engineers need to consider the HVAC system as whole and how each system component contributes. From that foundation, it is useful to examine the real benefits and a cost analysis of both vertical inline (VIL) pumps and base-mounted pumps.
The Footprint Factor
It is easy to become fixated on floor space when designing a hydronic system, especially if space is at a premium, but there is a common misperception in the market that the installed footprint of VIL pumps is much smaller than base-mounted pumps. Although this can be true in some smaller horsepower applications, such as boiler pumps (smaller hydronic system), it does not necessarily apply across the board. It is important to think about the footprint of the total installation, instead of solely the pump itself. Using this approach, any space advantage of installing VIL pumps becomes negligible.
Smaller VIL pumps mounted in the piping can create some space savings, but base-mounted pumps are the better overall choice for applications involving pumps larger than 10 to 15 horsepower (hp). For these larger applications, base-mounted pumps present a lower installation cost, greater energy savings, increased reliability, easier maintenance and smaller installed footprint. In fact, there are cases where base-mounted end-suction pump installations use less floor space than VIL, notably as the size of the pump increases. The VIL pump’s footprint is not the best choice for pump or drive combinations. The National Electrical Code (NEC) calls for 3 to 4 ft of clearance in front of the drive. This decisively negates any significant space advantage for VIL pumps in the context of the complete installation.
Accounting for Installation
While VIL pumps typically are a more cost-effective choice over base-mounted pumps, the nature of the VIL system dictates that more equipment often is needed to provide the appropriate pump support and vibration mitigation. An example of this is apparent in the ANSI Effect of Rigidity (Article 18.104.22.168.3.6): “ … Conventional horizontal split-case pumps (i.e., base mounted) are more rigid than vertically mounted pumps,” which means that they “become part of (the) structure (building).” Conversely, VIL pump installations are inherently flexible and may require additional support, such as isolation pads, flex connectors, heavier pipe flanges and piping to support the weight of the system. This additional equipment offsets the cost of the housekeeping pad for a base-mounted pump, and in some instances exceeds the cost housekeeping pad.
For pumps 7.5 hp or less, VIL installations can cost less than base-mounted pumps and in some cases are more practical. However, in midsize and larger pump applications, such as large chilled water pumps or condenser water pumps, base-mounted installations are more cost effective.
Ongoing pump maintenance is based on a number of factors, including mechanical seal replacement and whether or not a pump has true back pullout capability, enabling the pump to be serviced without disturbing the piping. An example of this is a centrifugal end suction pump that comes standard with a center drop out spacer coupling, which provides for bearing frame removal without the need to move the pump or the motor.
While it is true that mechanical seal replacement in VIL pumps might be considered easy, seal replacement is not total service. Total service also comprises replacement of the throttle bushing, impeller and pump shaft. The motor must be completely removed, requiring ample space and often cumbersome hoisting equipment even with a relatively small pump.
Depending on where the VIL pump is installed within the system, it may need to be removed and lowered from the piping at the ceiling to the ground for safe servicing. This process adds more maintenance downtime. To conserve space, VIL pumps tend to be spaced close together, which can pose problems for serviceability. It can be challenging, time consuming and pose a safety hazard for technicians to work on a large VIL pump between two others that continue to operate at full speed. One pump may need to be shut down, forfeiting redundancy and risking adequate capacity. Alternatively, service access in a base-mount installation is straightforward and safer because the layout of the system enables easier access.
The rotating elements within a pump cause vibration. Both base-mounted and VIL pumps have motors and impellers that rotate, and both pumps are designed and manufactured to the same vibration standard (ANSI/HI 9.6.4). Further, ANSI/HI 1.1-1.5, Centrifugal Pumps for Nomenclature, Definitions, Application & Operation, cites seven possible sources of vibration in pumps:
- Recirculation radial forces at low flows;
- Fluid separation at high flows;
- Cavitation due to NPSH problems;
- Air entrainment or aeration of the liquid;
- Hydraulic resonance in the piping;
- Solids contained in the liquid; and
- Wear of rotating components.
A closer review of the standard shows that base-mounted pumps are able to handle more vibration than VIL pumps (per ANSI 9.6.4-2000). Inline pumps send their vibration into the system piping and ultimately the building structure. Base-mounted pumps, typically attached to the ground, send their vibration into the earth, which can extend its equipment life up to twice that of VIL pumps (per ASHRAE 2015 Handbook and Department of Energy).
Vibration challenges must be managed based on the location of installation and horsepower required to fit the application. Inertia bases are necessary for pumps that are not installed on the ground floor to help absorb vibration. Base-mounted pumps installed on the ground floor only require a housekeeping pad. VIL pumps can be installed without an inertia base where the mass of the pumps, piping and water is relatively small and the building, ceiling and piping can accept the load, which generally is less than 7.5 hp. In larger horsepower applications (usually greater than 25 hp) a base-mounted pump is recommended with an inertia base (ASHRAE HVAC Applications Handbook section 48.44-45).
According to the ASHRAE HVAC Applications Handbook, in addition to inertia bases being required for most large horsepower applications not at ground level (i.e., base-mounted pumps), “all inline pumps should be mounted on free standing springs, regardless of location.” This point is worth noting, as there are mounting requirements for both types of pump.
Reliability & Life Expectancy
Base-mounted pumps have greater reliability, extending the product’s life. The mechanical seals in VIL pumps need external flushing tubes that are vulnerable to damage and can plug over time, leading to premature seal failures and lower pump life expectancy. Base-mounted pumps have internal flushing capability that passes up to three times more flow over the seal faces, helping reduce the seal temperature and extend seal life.
In addition, some base-mounted pumps have two sets of bearings: one set for the thrust and radial load and one set for the motor, reducing wear and tear. VIL pumps have only a motor bearing, which must do double duty, reducing life expectancy. By design, base-mounted pumps are built to last longer, as supported by the ASHRAE Equipment Life Expectancy Chart.
HVAC systems account for nearly 40% of energy used in commercial buildings in the U.S. Pumps play a major role in all HVAC systems, and due to the nature of their application, design and operation, pumps provide an excellent opportunity to reduce costs and increase reliability in HVAC systems. Building designers, owners and contractors are cautioned against specifying and installing VIL pumps simply based on purchase costs. To choose the best pumps for commercial building HVAC systems, there are several key factors to consider that affect performance, reliability and long-term ownership cost. The perception that VIL pumps save installation costs across the board is a myth. VIL pumps may be less expensive, and in some cases, more practical for smaller applications, but for midsize and large commercial installations, base-mounted pumps are more cost-effective and efficient.