Designs for pumping technologies

By making correct pump and piping selections, systems can operate at or near peak efficiency, avoid cavitation, and maintain a long service life. With each system type a multitude of options are available to a designer or engineer; but within these, the best option must be selected for a given application.

01/27/2016


This article is peer-reviewed.Learning Objectives:

  • Explain the types of pumps used in HVAC and plumbing systems.
  • Model calculations for pump speed, size, and curve selection.
  • Appraise energy-efficient solutions.

Figure 1: This image shows chilled-water heat exchangers (green pipe) on the left-hand side, which connects to a condenser-water circuit (blue/red pipe) for waterside economizer. At the back right are the associated floor-mounted chilled-water pumps and iWaterside pumping and piping systems are typically considered less complex and easier to apply/design than their airside counterparts. With greater density and heat transfer capacity, water can carry the same amount of energy through a much smaller conduit than air. This allows pipes to route through buildings with fewer clashes, and easier coordination with other trades within congested plenum spaces than the same energy transferred within ductwork.

A 2-in. chilled-water pipe is typically simpler to route through a congested ceiling space than a 30-in. equivalent duct (using a 16°F ΔT for chilled water and 20°F ΔT for air at 2.5 ft/100 ft pipe friction and 0.08 in. wc/100 ft.

With these arguments, people are led to believe that water systems require less effort in design and need less skill to implement than airside systems. These assumptions, however, hide the vast array of pumps available to choose, or the shortened operational life that can occur from improper engineering selections.

Pump construction and material types

A centrifugal pump is the standard pump type used within commercial buildings for both HVAC and plumbing systems. These pumps are made of a volute casing that houses the impeller, which rotates to draw water from the suction eye and discharge it radially out through the impeller vanes (perpendicular to the drive shaft). The impeller is attached to the drive motor either directly or through a separate drive shaft (closed- or split/flexibly coupled, respectively).

The water is directed out through the discharge nozzle with a very small quantity of water recirculating back at the cut water (resulting in pump inefficiencies as this clearance increases with reductions in impeller diameter for a specific volute casing size).

The volute and impeller can be constructed of differing materials depending upon the requirements of the system. The typical material types for different HVAC and plumbing systems are as follows:

Figure 2: This shows hot-water boilers and their associated insulated piping in the mainframe with the floor-mounted heating hot-water pumps and insulated piping on the bottom right-hand side. Courtesy: JBA Consulting EngineersHVAC: chilled, heating, and condenser water

Standard materials include a cast iron volute casing and a bronze impeller, referred to as a bronze fitted pump. These material types are typically only usable for nonpotable, treated water systems (controlled pH and chemical concentrations), where corrosion is unlikely, and more of the oxygen is removed from the system (either through standard air vents in a closed-loop system or through an air separator in an open-loop condenser water system). The typical pH requirements of chilled-, heating-, and condenser-water systems are from 7-9 (slightly alkaline), with total dissolved solids (TDS) less than than 1,500 ppm.

Review these values with the specific equipment to be used within the system, as they may vary depending upon the manufacturer. The TDS requirements are typically used for condenser-water systems to help define the specific cycles of concentration that are allowable for the water system. This is the number of times the water is able to recirculate through the piping, pumping, and cooling tower system before it is required to be drained out of the system and replaced with freshwater and further chemical treatment.

Plumbing: potable cold, hot, and hot-water recirculation

The standard material used in potable water booster pumps is stainless steel volute and impeller. This is due to the potable-water code requirements for low lead content within the system, as well as high levels of oxygen present within potable water and limited or no water treatment that is completed.

Though potable-water treatment in buildings is not common, some designs introduce chemicals such as chlorine dioxide, silver ions, or other means of controlling legionella bacteria beyond heating the system to approximately 140°F. Caution is recommended when providing additional treatment of potable water within commercial buildings. In many instances, these systems must be permitted and approved by the authority having jurisdiction (AHJ), as these systems can become a public health hazard if not properly implemented or maintained.

An item that may require additional selection and scheduling with a pump is the respective mechanical seals. The mechanical seal provides the watertight seal between the volute casing and the motor drive shaft where it must connect to drive the impeller. Pump manufacturers provide a standard seal with each pump, but other options are available from the factory. Seal types can be as simple as a stuffing box packed with coated cord until it seals the opening between the casing and drive shaft. These are typically only used for fire-sprinkler water pumps or in some cooling tower applications where high levels of TDS are present. The other typical mechanical seal uses a stationary seat and a rotating seal ring pressed against each other by a spring along with an O-ring elastomer.

There are various types of materials available for these two parts, depending upon cost and type of water in the system, from carbon/ceramic to silicon carbide/silicon carbide (SiC/SiC). SiC/SiC seals are effective with higher levels of total dissolved solids, abrasive resistance, and higher-temperature water systems (when paired with the correct O-ring, Buna, Viton, or EPDM [ethylene propylene diene monomer rubber], as required for the temperature of liquid).

One side note to mention regarding centrifugal pumps is the misconception of force when talking about pump operation and water flow. The "centrifugal force" that is often talked about in pump books and literature, which is stated as the force keeping an object following a curved path from moving away from the center, is actually a fake force in physics terms. If you remember your physics classes, centrifugal force is actually the result of the inertia of the object trying to resist the change in movement. The actual force attributed to this phenomenon is centripetal force, which is the force that keeps an object in a rotational path instead of moving in the vector direction of velocity at a specific point. Centripetal force acts in the direction perpendicular to velocity, between the object and the center of rotation.

Figure 3: Split-case pumps are used within a central plant condenser-water system. Courtesy: JBA Consulting EngineersPump selections

Choosing an HVAC pump for a specific duty point has been well covered in previous articles such as "Selecting an HVAC Pump" (May 2015). The article includes calculations for expected system pressure drop (total dynamic head) along with the flow requirements based upon the heat-capacity requirements. Please review the article for detailed information regarding net-positive suction head (NPSH available of the system greater than the NPSHR [required] of the specific pump), as well as calculating friction loss in a piping system for both closed- and open-loop systems.

Closed-loop systems: These typically are HVAC chilled- and heating-water piping systems that are not open to atmosphere. The required pressure loss of the piping system takes into account only the full piping loop (distance to the farthest coil multiplied by two for supply and return piping) along with all of the equipment and accessory losses in the system including valves, coils, strainers, etc.

For large piping systems, the friction loss through elbows is typically considered as an additional 25% to 35% of the straight length of piping (125% to 135% total straight piping). For small systems, this value can be up to 75% to 100% of the straight pipe length (175% to 200% total). The physical elbows can be counted as well, but, as engineers know, most piping designs are not installed identically to the contract drawings.

Using percentages from the straight length provides a gauge of the overall system schematic, but entry-level designers/engineers should calculate the full pressure drop elbow by elbow and piece by piece for a few systems before they can be comfortable with using percentage rules without issues.

The standard pressure drop and velocity used to size closed-loop piping systems are 2.5 ft/100 ft and a maximum of 8 fps (whichever is more stringent for a pipe size). These are rules of thumb, but are based upon empirical data from previous system designs. The friction-loss limitation assists in maintaining flow at the farthest fixtures and maintaining velocities below the threshold for acoustical requirements where piping is located adjacent to noise-sensitive areas (as defined by acoustical-consultant reports. and the limitation is used for more standard commercial buildings, areas like theater seating may require additional flow restrictions).

These values can be exceeded when present in dedicated mechanical rooms or where the respective terminal unit is located close to the pump discharge, which allows for increased pressure available in the branch circuit. Expected system diversity also should be taken into account, especially for large systems where all areas of a facility should not be occupied simultaneously. This allows the main utility piping to be reduced in size from the theoretical combined flow rate, as only 80% may be used even when the facility is fully loaded. 

The static height of the water column (for piping serving towers or high-rise buildings) is only accounted within the working-pressure requirement of the pump. The total dynamic head of the system, along with expansion-tank precharge, is also added to the static height of water for the pump's working pressure. The typical pump working pressure is 150 psi as standard with 300 psi available as an option in most cases.

An important item to be mindful of when designing a closed-loop piping/pumping system is the inclusion of an expansion tank to compensate for fluid expansion between system fill and equipment operation. This is important for both chilled-water and heating-water systems, as there is always a temperature differential between the filled temperature of the system and the operational temperature. This is typically the difference between the groundwater temperature and the maximum temperature expected in the system.

For an area such as Las Vegas, the groundwater fill temperature is approximately 60°F with a maximum of 85°F for chilled-water systems (when the chillers are not operational and the system is under flushing or the branch is stagnant during summer) and upwards of 180°F for heating-water system supply temperatures with a standard noncondensing boiler system. The precharge pressure of the expansion tank will add approximately 15 psi to the system pressure to maintain enough pressure at the top of the system for correct flow through air handling unit (AHU) coils and other heat-transfer devices that may exist at the farthest point in the system.

Pumps used within closed-loop systems include vertical-inline, end-suction, and horizontal/vertical split-case, though we have recently seen vertical-inline pumps used more frequently in chilled- and heating-water systems due to their lower vibration and floor-footprint requirements within central plants. These include small, medium, and large central plant heating-, chilled-, and condenser-water pumps.


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