Over the Top?

Much has been written in recent years on the subject of energy efficiency.

Within the HVAC industry we are all now familiar with topics we had never heard of a decade ago such as Life Cycle Costs, EFF1 motors, Enhanced Capital Allowances and the myriad of other arguments that have been developed in recent years to support and encourage specifiers within the industry to select energy efficient variable speed pump technology over their less efficient fixed speed counterparts.
However, there is one subject that is rarely either considered or discussed but that is an even more expensive solution than simply running a fixed speed pump 24/7 for all its lifetime and that is pump over-sizing.

It is a fact that 95% of all the installed base of pumps are oversized by at least 20%. Add to this the knowledge that most pumps only need to work at 100% duty for approximately 5% of their operating life and it becomes easy to see that a huge amount of energy is being wasted and this is not happening in the frequently discussed benefits of specifying variable speed options.

So how does over-sizing happen and is it really that common? The answer to this is yes but that over-sizing tends to rather creep in to a specification as opposed to being purposefully engineered in, but how? When calculating the flow and head to size a pump, without even thinking about it a word creeps into the equation, and that word is ‘allow’.

This allowance factor has become the accepted third contributor in the pump sizing equation– for example a 20% allowance for an increase in head would be quite standard, as would a 10% allowance to the flow calculation.

All of these allowances contribute to a larger pump being specified than the system actually requires and that then means a more expensive pump is purchased than was necessary. Of course the real problem comes when you get into the energy costs associated with an oversized pump over its lifetime as this factor accounts for 80%+ of the total cost of the pump when you calculate its ‘cradle to grave’ cost.

It is true to say that generous pump sizing can be compensated by ‘throttling’ back at commissioning stage but this principal of engineering in and then taking it out again is a very expensive way to get to a solution that isn’t the ideal.

Pump Affinity Laws predict the affects of changing the speed of a centrifugal pump and show the clear affinity between RPM, Flow and Head which result in Effect (power consumption).

The actual result of this is that with just a 20% reduction in speed the electrical saving is 50%. If the speed can be reduced by 50% the equivalent electrical saving is a massive 87.5%!

So getting it right initially offers incredible benefits both for the consultant, contractor, end-user as well as the environment.

But getting it right first time has even more ramifications as often pump solutions within commercial applications stay in situ for 25+ years. Therefore it is even more critical to correctly size pumps for all HVAC applications as when it to comes to having to replace a pump the initial over-sizing is repeated again and again.

Generally in retrofit applications little consideration is given to any changes in system demand over time, and original duties are therefore replaced like for like. This situation is bad enough but when you add to this the reality that older model fixed speed pumps are being replaced with fixed speed counterparts and not the energy efficient variable speed options as would happen in best-case scenario. Additionally with this like for like replacement, no account is being taken of the improved efficiencies – that could mean that a pump with a smaller motor would be more than adequate for the job which would offer both capital savings as well as ongoing energy savings.

Indeed there may be many other aspects that would dramatically affect the demands placed on the pump that should also be considered during the replacement phase.

There are many tools available that will help you to achieve the optimal solution. Grundfos have available either a windows based CD called WinCAPS or an on-line version called WebCAPS both of which offer computer aided product selection. To decide which option is best for your particular project simply visit www.grundfos.co.uk to request your CD or to use the on-line version.

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