Sizing, selecting pumps and circulators
By making correct pump and piping selections, a hydronic system can operate at or near peak efficiency, avoiding cavitation and vibration. Here’s a look at relevant codes and best practices.
- Review codes and standards that guide design and specification of pumps and piping systems.
- Become familiar with different types of pumps and their function as related to an application.
- Understand basic information for selecting pumps to meet a distribution loop’s requirements.
- Understand key equipment and its integration for energy efficiency in HVAC systems.
There are a number of pumps manufactured today that can be used for different applications. The most typical application is circulating and distributing chilled or hot water for a variety of different building or facility load requirements. The process of selecting and sizing pumps and circulators includes several steps that a designer needs to take to complete his or her task for any given installation. The concepts to consider for a particular design task and the designer’s experience level will determine the complexity of the overall process. Sizing and selecting pumps is not that difficult once the experience has been gained. At the very least, the designer must:
- Define and understand the system application, and perform a hydraulic or fluid system analysis
- Determine the basic pump (or circulator) and driver type for the application
- Determine the pump size and its design operating point
- Decide on pump design considerations to maximize the system’s energy efficiency.
Designers must understand many basic concepts regarding pumps and hydronic systems, but for this article the fluid systems discussed will be water only.
Codes, standards, and guides
The design of a hydronic system includes multiple components related to the application that require a review of codes, standards, and/or regulations necessary to complete the design and avoid conflicts that will cost time and money to resolve. Local, state, and federal codes and/or regulations dictate requirements that may affect the design, but there are a few related codes targeting pumps which refer to the specific application.
Organizations such as ASHRAE, the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code, the American Petroleum Institute (API Std. 610), and UL provide several code references or have standards to review for systems, equipment, and testing requirements. ASHRAE Handbook HVAC Systems and Equipment – 2012, Chapter 44 provides information on pumps. ASME PTC 8.2-1990 for Centrifugal Pumps provides information on pump testing and guidance for the application of the code related to centrifugal pumps. UL provides references to the use of pumps related to the systems to which they are applied such as UL 343 - Standard for Pumps for Oil-Burning Appliances, or UL 778. Standard for Motor-Operated Water Pumps. In addition, the Hydraulic Institute (HI) has a vast library of standards related to pumps that the designer will find very helpful, such as Rotodynamic (Centrifugal) Pump Applications.
A good technical resource for most facility engineers is ASHRAE. ASHRAE has numerous technical sources of information including a series of four handbooks that are updated every 4 years. Each handbook has an entire chapter dedicated to listing “Selected Codes and Standards Published by Various Societies and Associations” relevant to the topics covered within the handbooks. Additionally, ASHRAE Standard 90.1-2013: Energy Standard for Buildings Except Low-Rise Residential Buildings is the reference standard for energy efficiency. This standard illustrates minimum efficiency requirements for building envelope, HVAC, power, lighting, and other equipment. Chapter 6, HVAC, is where designers will find minimum energy efficiency requirements for HVAC system construction with listings for component items such as: water- and/or air-cooled chillers, piping system design flow rates, pumps, insulation, and controls.
Define and understand the system application
A pump is required in a system composed of piping, fittings, heat exchangers, and other equipment through which a fluid needs to be delivered or transferred. This delivery may be for a long distance, to higher elevations, or circulated within a pressurized loop to assist in a process and perform work. The pumping requirement is immediately affected by the fluid’s nature and its properties of viscosity, density, and specific gravity. The discussion of these topics, although important to the designer, is beyond the scope of this article; however, these terms, along with vapor pressure and the effects of temperatures on a fluid, should be considered in many applications. Additional “pumping” terms will be introduced as we progress and some basic definitions will be provided.
Pumps as discussed in this article are generally considered to be larger with a larger drivers (i.e., motors), usually pad mounted or otherwise mounted to a floor assembly, and can be used in both closed- or open-circuit systems such as a condenser water loop with cooling towers. A “circulator” is a pump but is typically used in a closed circuit, and is usually smaller with a fractional horsepower motor, although that is not always the case. A circulator only needs to overcome the frictional losses in a piping system without changes required in elevations. These smaller pumps are often sealed units, with the motor rotor, pump impeller, and components such as bearings all sealed within the fluid circuit. Because they are typically smaller, they can be supported entirely by the piping system (flanges). The designer will undergo similar steps in sizing and selecting circulators as with pumps.
Along with the fluid, other items that affect the overall system design and thus the pump selection include: equipment layout, flow paths, the size and length as well as the type and age of the piping, fittings, valves, piping specialties or appurtenances, noise, and any elevation changes. These parameters establish related system friction losses or pressure drops. Determining this “resistance to flow” in a new or existing hydronic system is probably the most important design task to be completed. All units will be expressed as English units here. This friction pressure drop is referred to as “friction head” loss (Hf), and is usually expressed in height of a liquid column in feet.
One method of calculating the friction head loss is the Darcy-Weisbach formula:
The length (L in ft), inside diameter (D in ft), and a dimensionless friction factor (f) of the pipe are used, along with the “velocity head” (Hv) in feet using the fluid flow velocity (V in ft/sec) and the acceleration of gravity (g or 32.2 ft/sec2) to calculate this pressure loss. The friction factor of the piping (f) considers the relative roughness of the piping and the “Reynolds Number” based on the pipe diameter and the fluid properties (viscosity, density, specific gravity) and velocity. In some circumstances, the Williams-Hazen formula can be used. The designer should review these formulas, terms, and concepts to fully understand their importance.
Calculating pressure losses through a piping system for the required head of a pump selection can be done easily enough with an automated spreadsheet using the concept of equivalent lengths of pipe and determining the pressure loss per 100 ft of piping. This method is similar to completing HVAC ductwork friction loss calculations. The system information or items listed above are required for a designer to complete the calculations, plus he or she must determine a factor of safety to use. The Cameron Hydraulic Data suggests 15% to 20% safety factors for commercial piping systems. Head loss per 100 ft of pipe due to friction can also be found in Cameron Hydraulic Data friction loss tables for pure water at 60 F and clean, new pipe.
Additional adjustments or corrections are required for other temperatures and conditions, and friction can vary with temperature and pipe roughness. The spreadsheet can calculate total dynamic head by multiplying the total equivalent length of a pipe segment by the head loss per 100 ft of pipe. The designer will need to look up equivalent lengths for all the fittings based on the type selected. The loss from each pipe segment is summed and then the safety factor is applied. The total dynamic head can also be rounded up to the next 5 (adjustable) ft of head.
The majority of designers today use a computer program to perform some type of hydraulic modeling of the distribution system to calculate the pressure losses. These head loss calculations, however done, should be completed for any design as they will dictate the selection of all component equipment (chillers, pumps, etc.) as well as the pressure class of the distribution piping, fittings, and valves within the system. These pressures will, in turn, be related to the selected pumping scheme. The purpose is to try to balance the system under design flows including any parallel flow paths, and determine the required pump head to overcome the losses. The overall calculation process is iterative, especially for a newly designed system. An existing system being modified might require several modeling runs as well, but may require only a few changes to fully integrate the system to the modified use. The main point to remember is that every component within the system will affect the pressure and the fluid flow rate and will either fix the pressure at a particular level, increase the pressure, or decrease the pressure.
After the system is defined, which might also include a simple process flow diagram (Figure 1) or a more detailed piping and instrumentation diagram (PID), and the head losses are determined, the designer should develop “system head curves” (discussed later in the article). These curves will correlate the volumetric flow rates through the various flow paths to the related pressures or hydraulic losses that will be experienced in the piping system.
Additional items to consider for a complete system understanding are:
- Whether the system will be operated continuously or in an intermittent mode
- The system’s flow type (e.g., constant or variable volume)
- A need to provide continuity of service or redundancy for any equipment (N+1)
- A need for future capacity or expansion of the system