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It’s no secret that effectively designedvariable speed pumping can savemoney and energy in a wide variety of applications. But manyconsulting/specifying engineers aresurprised to learn that it can also signif-icantly reduce thermal contamination inchilled water systems.

In simple terms, thermal contaminationis the addition of unnecessary,unwanted heat to chilled water. Due to

basic laws of physics, every time waterflows in a pipe, water friction occurs inthe form of pipe loss and system headloss, resulting in thermal contamina-tion. This phenomenon occurs everytime chilled water is pumped. SeeFigure 1.

Some of the heat is added in the pumpitself. More is added through the fric-tion of pipe and system head loss as

water droplets collide with eachother and with pipe walls as theytransit the piping system to and fromthe pump. Additionally, before waterreturns to the chiller, it is exposed toeven more thermal contamination bythe chiller pump.

But by expending less energy, chilledwater systems can significantlyreduce thermal contamination. Sinceone pump horsepower equals 2,545.8

BTUs per hour or 42.43 BTUs perminute, and one ton of refrigerationequals 12,000 BTUs per hour, we canrelate pump horsepower (or energy)savings to BTU (or thermal)contamination within a system.

Eliminating Costly Thermal Contamination In Chilled Water Systems

See Chilled Water Systems, pg. 2

Figure 1. Simple Chilled Water Circuit

Volume 18 Issue 1 October 2003

■ BELL & GOSSETT■ DOMESTIC PUMP■ HOFFMAN SPECIALTY■ McDONNELL & MILLER ™

Fluid Handling

Bell & Gossett Packaged Systems GroupVariable Speed PumpingHeat TransferPressure Boosting

True unit responsibility and optimized performance from the system experts.

Electrical components, pumps, and piping all factory assembled and tested.

installation.

When space is limited, individual matched components can be shipped for field

Contact Wallace Eannace Associates, Inc. , your local B&G Representative, for assistace

in optimazing your pumping system to meet its requirements and reduce energy use.

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For instance, if 100 horsepower issaved in pumping, a reduction in totalplant heat or thermal load of 21.2 tons

of refrigeration can result.That’s more than five percent of a400-ton chiller or 10 percentof a 200-ton chiller.

With the exception of a smallamount of power lost in thepump bearings and the stuff-ing boxes (termedmechanical losses), the dif-ference between the brakehorsepower and hydraulichorsepower represents

energy degradation withinthe pump itself. Most of thisis transferred in the form of heat to liquid passingthrough the pump.

The mechanical losses of agiven pump depend on the size of thatpump and the relationship of its bear-ing load and mechanical seal diameter.You can estimate what these mechani-cal losses are. Figure 2 reflectstypical mechanical losses for centrifu-gal pumps rated from 50 to 10,000gpm. The mechanical losses areexpressed as the percentage of totalshaft horsepower.

For instance, on a 1,000-gpm pump,the total mechanical losses on a 50horsepower rating would amount toonly about two percent - or about onehorsepower on a typical centrifugalpump. The remaining 49 horsepowerwill contribute to the thermal load andcontamination of the system.

In many cases, the heat per se added bythe pump to the fluid stream may notbe great and represents the inefficiencyof the pump itself. However, the pumpimparts greater energy into the chilledwater in the form of pressure.Although it is not thermal contamina-tion yet, as the chilled water istransferred throughout the circuits, therubbing and friction leads to signifi-cant thermal contamination.

But much of this wasted energy can besaved through variable speed pumping.In addition, system energy expendi-tures and flows can be reduced orincreased to meet actual needs.

Using a common example, 100 percent

of full load design operation is onlyrequired 2 percent of the time, whereas50 percent of the full load is required

30 percent of the time.While the pumping systemmust be designed for fullload operation, the systemoperates at only half itscapacity 15 times longer

than at full capacity.As system loads decrease,so does the required flow.As flow decreases, so doessystem resistance or fric-tion.

In a given system, frictionhead loss varies as thesquare of flow, or Head 2 =(Capacity 2 / Capacity 1)2 x Head 1

With this formula, a headloss curve can be estab-lished. This is shown inFigure 3 as the system headloss curve or variable headloss. The pump control curve isderived from the system head loss plusthe addition of the design pressuredrop.

The shaded area depicts constant speedpump performance overlaid on the

actual system control curve and iswasted pumping energy or direct ther-mal contamination.

However, variable speed pumpingtracks the control curve. Thus, only theflow actually required at any given

time is pumped. This action substan-tially lowers annual operating costs.Since variable speed pumping tracks

true demand, the annual savings,when compared to safety factored oroverheaded pumps, is even greater.

It’s easy to quantify direct savingsfrom using variable speed pumpingto reduce thermal contamination. Amodern chiller typically will costapproximately $300 in initial capitalcosts for each ton of refrigeration itproduces. So initially reducingrefrigeration needs by 10 tons saves$3,000. At .063 average kilowattsper ton and at a 10 cents electricalenergy cost, that same 10 tons of refrigeration saved will return over$5,500 per year in energy costs.

With numbers like those, it’s hard toignore thermal contamination and thebenefits of system responsive variablespeed pumping.

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P ROBLEM S OLVING

Chilled Water Systemsfrom pg. 1

Figure 2 — Typical Mechanical Losses

Figure 3 — System Energy Needs Vary WithLoad/ Wasted Pumping Energy

© Copyright ITT Industries, Inc., 2003