Pump Sizing Basics

An important part of a mechanical engineer’s job is to select the correct pump for any given application. Be it heating, chilled water, boosted domestic water or hot water return pumps (and many others). Here we explore the various factors an engineer must consider when making pump selections and include some guidance notes for reference.

Pump Duty

The performance of a pump is called it’s ‘duty’. The duty comprises of flow rate and head pressure. This is basically a measure of how much liquid it can deliver against what resistance. The flow rate is commonly measured in litres per second (l/s), meters cubed per second (m3/s) or Gallons per minute (GPM). The larger a system is, the larger flow rate is required.

The head pressure of a system is a measure of how hard the pump needs to work to achieve the desired flow rate. In general, the longer the pipe lines, and the more bends and fittings, the higher the head pressure required.

Modern pumps typically have variable speed functionality. If controlled correctly, variable speed pumps can adjust their output based on system demand, to minimise energy consumption.

For more in depth guidance on calculating pump duties, please see the Building Services Portal pump sizer. Once the required pump duty is established, you will need to select one from a manufacturer’s catalogue. For any given duty, there maybe several options for pump selection, so below is some guidance for where to start selecting which type of pump for common applications.

Pump Types

There are many different types of pumps, we will go through the most commonly used for the the various application types. This may help with pump selection and there are also some tips for each application;

Application

Domestic Heating Systems- residential homes, flats, houses, small commercial properties

Pump Type

Single canned-rotor type pumps

domestic heating pump 1

Selection Notes

Selection of domestic heating pumps is typically based on several factors, including the system’s approximate size and the plumber’s expertise & experience. However, these selections often rely on a limited range of standard-sized pumps and may not prioritise precision. Unlike commercial systems, domestic systems are typically not meticulously balanced. In such cases, users themselves make adjustments according to their specific requirements by tweaking radiator valves and pumps speeds to achieve optimal performance.

Application

Domestic Heating- Heat Pump Systems

Pump Type

Single canned-rotor type pumps

Domestic heating pump

Selection Notes

Similar to the aforementioned domestic heating systems, these systems share certain similarities. However, one key distinction lies in the fact that, typically, larger flow rates are necessary due to the reduced flow temperatures. As a result, meticulous attention must be given to pump selection in these cases. It is crucial to calculate the pump duty accurately, and recommend using guidance such as the Building Services Portal pump sizing.

Application

Small Commercial Heating Systems- for example: retail properties, care homes, schools, hospitals, leisure centres, offices, large residential properties.

Pump Type

Twin head canned-rotor or inline type pumps

twin head small commercial

Selection Notes

Typically, smaller commercial heating systems could be served by similar pumps to domestic types depending on the system size. Often, it is beneficial to select twin pumps so that there is a back-up should one pump fail. A back-up pump can also be achieved with two single pumps installed in parallel with automatic change over controls. Pumps can take the form of canned-rota or inline type. Commercial pumps should have there duties carefully selected with such as method as found here, Building Services Portal pump sizing.

Application

Large Commercial Heating Systems- for example: multi-storey offices, ware houses, hospitals, large schools, leisure centres, district heating systems, Universities.

Pump Type

Large in-line pumps/ end suction pumps

Selection Notes

Larger commercial heating and district heating systems should have pump duties carefully calculated. This can be carried out with the same Building Services Portal pump sizing method as used for smaller systems. Larger systems tend to have pumps with larger connection sizes, motors and inverters, such as the in-line and end suction types pictured. Larger pumps can require mounting on concrete plinths or even inertia bases. Anti-vibration mounts and flexible connections may also be required. The mounting requirements will be specified by the manufacturer. Most commercial pump are supplied with inverter controls, which enable the pump output to be modulated down to match the required duty during commissioning.

Application

Chilled Water Systems

Pump Type

In-line pumps/ end suction pumps

Selection Notes

Chilled water systems typically use similar pumps to commercial heating systems. Chilled water system tend to operate on smaller temperature differences, and therefore tend to have higher flow rates than their heating system counterparts for any given building. Engineers must ensure chilled water pump selections are compatible with any anti-freeze dosing in the system, such as glycol. Pump duties can also be calculated using the Building Services Portal pump sizing method.

Application

Boosted Cold Water Service- residential mains water pressure boosting, commercial domestic water pressure boosting

Pump Type

Single-stage/ multi-stage centrigual pumps

Selection Notes

Typically domestic water pressure boosting is required for large multi-storey buildings, where mains water pressure is not sufficient to serve higher floors. Boosted cold water systems tend to be paired with water storage tanks and pressure vessels. Often supplied as packaged systems that include duty/ assist/ standby pumps, pressure vessels, inlet & outlet headers and controls. 

Calculating the head pressure required for booster pumps is different from circulation pups in that vertical height and outlet pressure required must be factored in. The Building Services Portal pump sizing method can be used to calculate required pump duty for these systems.

Application

Hot Water Secondary Return Pump

Pump Type

Single canned-rotor/ in-line type pumps with bronze or stainless steel housings

Selection Notes

Hot water secondary return pumps are used to circulate hot water back to calorifiers in order to prevent low hot water delivery to outlets and prevent stagnation of warm water (which can lead to issues such as legionella growth). Pumps need to be suitable for contact with raw water and therefore typically have bronze or stainless steel housings/  internal components. Sizing of hot water return circuits should be based on the calculated heat loss in the hot water system, and usually are based on keeping hot water temperature above around 55 deg C.

 Application

Drainage Pumps

Pump Types

Submersible sump pumps/ macerators/ waste water lift pumps

Selection Notes

Various types of drainage pumps have been listed her, which building services engineers are likely to select.

Sump pumps typically are used in basements where accumulated water  is below the level of the gravity drainage system, and therefore a pump is needed to lift the water high enough to connect onto it. Selection needs to be based on required flow rates and vertical/ horizontal distance to the gravity drainage connection.

Macerators and waste lifting pumps are typically used where sanitaryware needs to be installed remoted from soil stacks/ underground drainage connections. Selection is normally based on the connected sanitaryware and vertical and horizontal distance to the gravity drainage system connection.

Reading Pump Performance Curves

All pumps have will have a performance curve associated with them which we can use to determine if they are suitable for the required pump duty. Performance curves are a plot of available head pressure of the pump, against the flow rate being delivered.

Example 1

A flow rate of 1 litre per second is required at a head pressure of 150 kPa. This point has been plotted on the selected pump curve. As we can see the point in red sits below the curve, which suggests the pump is capable of delivering the required duty.

Example 2

Using the same pump, and therefore the same performance curve as example 1. This time a greater duty is required of 1.25 litres per second at 200 kPa. This duty point has been plotted on the curve. As you can see, the duty point sits above the curve and therefore the pump is not capable of delivering the required duty. In this instance, a more powerful pump will need to be considered.

Try the Building Services Portal Pump Sizer now for free, the quick, easy and accurate pump sizing method. Link includes the Building Services Portal pump sizer tutorial.