Choose the right pump for the job

Pumps are normally specified with a safety factor of about 10% excess head at design flow rates and/or 15% excess flow at design pressure loss. However, specifiers often select the next higher pump curve, resulting in a pump that is 15% to 20% oversized. While this selection will ensure that the oversized pump will provide sufficient flow, there are also problems associated with overspecification.

The costs incurred through installing oversized pumps can be considerable. The purchasing cost of the pump and the motor assembly will almost certainly be higher – the general rule being: the bigger the pump, the higher the price. Additionally, the associated fittings and controls for an oversized pump may be larger and consequently more expensive to purchase and fit. Maintenance and running costs will be higher, as an oversized pump operating further away from bep will experience greater stress and will use more energy.

The UK government offers the following advice to specifiers in Building a better quality of life – A strategy for more sustainable construction: "Design out waste both during construction and from the useful life, and afterlife, of the building or structure. Think whole-life costs. Involve the supply chain. Specify performance requirements with care to encourage more efficient use of resources.

"Waste does not imply just unwanted physical material. It includes unnecessary consumption of land, time, lower than planned economic return and unrealised potential from built assets during their lives. Minimising waste through design means avoiding overspecification of materials and services in favour of simplicity (build ability), bearing in mind operation and maintenance (maintainability), and considering flexibility and future re-use (adaptability), so as to minimise construction costs."

Getting the priorities right
It is unfortunate that despite the additional cost in money, time and energy associated with oversized pumps, many specifiers still prioritise initial performance over system life cycle costs.

While it is general practice to select pumps for a duty corresponding to maximum design load, it is a fact that when the practical operation of the heating system is taken into account, pumps operate at a much lower load factor, or part-load duty for the majority of the time.

The 100% load situation (external design temperature depending on the temperature zone) occurs very rarely. Therefore, the temperature drop ?T given for maximum load on the system at the duty point ( ?T=20ºC) is never realised. Furthermore, the actual heat loss of a building is almost always lower than the system's maximum calculated capacity, due to the safety margins included in the heat-loss calculation.

Another factor to consider is that individual radiators, as well as whole sets of radiators, are often turned down or off at the slightest rise in external temperature. As a result the system operates mostly under partial load conditions.

Figured it out
The relationship between the actual and calculated heat losses under variable exterior temperature (t_a) and constant interior temperature (t_i) is given by:

? = t_i - t_a /t_i - t_a min. Where ? is the load factor of the system, t_I is the design room temperature, t_a is actual external temperature, and t_a min is design external temperature.

Using an average heating season of 226 days per year, it can be seen that the average load factor lies mostly between 0·5 and 0·8. The load factor valve level rarely reaches 1·0 (100% load).

It follows then, with lower heating load and subsequently reduced flow rate (due to valve throttling) or oversized system load, that it is vital the system should be adaptable to match the flow rate of the heating media with the actual load demand.

The diagram bottom left of figure 1 illustrates that combinations of maximum pump capacity, high-power consumption, and design load on the system only occurs during a few days of operation per year. For example, when a standard multi-speed glandless circulator is used, on the majority of operating days, the lowest pump capacity setting is adequate and offers an energy saving of approximately 50%.

Confusing characteristics
Any confusion surrounding the correct specification of pumps can be attributed to the influence of radiator characteristics. The diagram on the left demonstrates that a reduction of approximately 30% in flow rate will have a negligible effect on the heating capacity of a radiator.

Take another example: a reduction of 15% in the flow rate will result in a fall in heating capacity of just 3%. In a practical situation, this loss will be compensated by external heat sources such as lighting, household or office equipment. Conversely, an increase of 15% in flow rate leads to an increase in heating capacity of around 2%, due to the decreasing slope of the curve with increasing flow rates.

Small increase
The diagram above left shows that in the area above the point of standard operating conditions, hardly any increase in heating capacity can be achieved – even when the flow rate is increased considerably.

Under partial load, as is the case on 98% of heating days, the changes in heating capacity of the radiator, caused by reductions in flow rate, are smaller still. This is because the slope of the radiator characteristic curve decreases as flow temperature falls.

In an unchanged radiator with a lower flow temperature (T_v=60°C, T_r=50°C, ?T=10°C) the heating capacity at maximum volumetric flow is 50% of full capacity under standard conditions. In this example, a reduction of 15% in flow rate brings a 1% or 2% reduction in heating capacity.

Generally, it can be assumed that a reduction in flow rate of 10% to 20% will have no effect on the heating capacity of the system when it operates under partial load.

Help is out there
The problems associated with overspecification have not only given rise to a new generation of energy-efficient, variable-speed drive pumps, but also to expert system computer programs, designed to make selecting the right pump easier and quicker.

Despite these advances, however, specifiers and contractors continue to overspecify, seemingly unaware that it is less expensive to buy and run the right pump than an oversized one. Increasing awareness of the cost involved in specifying oversized pumps should hopefully discourage this industry-wide tendency.