Variable-speed pump economics

by Sally Bouorm | April 1, 2011 9:36 am

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By John Ott

When a service professional tells a swimming pool owner they can cut energy costs by installing a variable speed pump (VSP) they become interested. However, when they tell them the pump will operate two-to-three times longer than an average pump, they are aghast. It is understandable for a consumer to question this line of thinking. With that being said, this article provides information that can be shared with customers in order to help them make the right decision when installing a VSP.

Reviewing the Affinity Laws

Reducing the speed of a pump’s impeller by half does not reduce electricity costs by the same 50 per cent. Rather, energy costs are slashed an amazing four times as much as the reduction in motor speed. This is accomplished through physics known as pump Affinity Laws. These laws are used in hydraulics to understand the relationship between several variables involved in pump performance (e.g. head, volumetric flow rate, shaft speed and power).

Therefore, a pump operating at half (1,725 revolutions per minute [rpm]) of full speed (3,450 rpm), the system will have one quarter the head pressure and use approximately one eighth of the energy (actual savings may be slightly less since the impeller is typically optimized for a particular speed).

Of course, running a pump at half speed may double the time it takes to completely turnover the pool water; however, doubling the run time only uses a quarter of the energy the pump would require running at full speed. This results in a 75 per cent energy savings.

Variable-speed benefits

Having a VSP allows homeowners to further save on electricity costs by simply reducing the pump’s speed by half again (approximately 863 rpm). In other words, every time the pump’s motor speed is reduced, the Affinity Law applies again.

On average, a VSP at capacity will draw 2,650 watts (this will vary depending on motor type) at 3,450 rpm and pump approximately 625 litres per minute (lpm) (165 gallons per minute [gpm]). This is an estimate, as lpm/gpm will vary between different manufactures at low head pressure (actual lpm/gpm will vary with the system head pressure). At 1,725 rpm the pump will draw 375 watts with about 310 lpm (82 gpm). This is an astounding saving of approximately 87 per cent.

Another way to look at these savings is in watts per gallon/litre. At 625 lpm (165 gpm) and 2,650 watts, the pump would run at approximately 4.2 watts per litre (16 watts per gallon). At the pump’s lower speed, which produces 310 lpm (82 gpm) at 375 watts, it would run at approximately 1.2 watts per litre (4.5 watts per gallon).

For example, installing a VSP on a 94,635-L (25,000-gal) swimming pool would yield the following results:

• At 3,450 rpm, the pump will draw 4.2 watts per litre (16 watts per gallon) equaling 400,000 watts consumed.
• At 1,725 rpm, the pump will draw 1.2 watts per litre (4.5 watts per gallon) equaling 112,500 watts consumed. A savings of approximately 72 per cent.

If the pump’s speed were reduced further, for example to 1,000 rpm, the result would be 189 lpm (50 gpm) at 95 watts. Using the same 94,635-L (25,000-gal) swimming pool, the result would be 0.5 watts per litre (1.9 watts per gallon) for a total of 47,500 watts. This represents an approximate saving of 88 per cent in terms of energy consumption compared to the pump running at 3,450 rpm.

Slow and steady

There are many other benefits that come from utilizing a VSP and pumping water slower, or at a specific flow rate, such as:

• Each piece of pool or spa equipment that has a recommended flow rate for operation can be set for a specific speed and time;
• Running water slower through the filter will typically capture smaller particles;
• Better distribution of chemicals since the pump is running for a longer period of time;
• Reduced Total Dynamic Head (TDH) helps decrease equipment wear and tear; and
• Quieter operation.

Installation tips

When installing a VSP on a new swimming pool, care should be taken by builders to ensure proper pipe and equipment sizing is used to handle the pump’s required maximum flow rate. For existing pools, it is important to verify the pump’s maximum flow rate will not exceed the capacity of the existing circulation system and equipment. However, keep in mind the pipe length between the pump’s suction port and first elbow or fitting should be at least five times the pipe size.

Programming tips

It has already been stated that running the pump for longer periods of time at the lowest speed will save the most money.

When determining the minimum and maximum speeds for a given system the guidelines are easy. For maximum speed, cavitation (the loss of pressure in flowing liquid, which can damage moving parts) can be beneficial.

Ramping up the VSP’s speed until the pump cavitates can help identify its upper limit of speed. The lower limit should be able to run any piece of equipment that requires a specific speed during circulation, e.g. salt cell generator requirements. It is also important to make sure the filter fills completely with water to avoid filtration problems. To verify this, make sure water comes out of the filter’s air relief valve.

 

John Ott has been with Hayward Pool Products in Pomona, Calif., for 26 years. His experience in the swimming pool industry expands 37 years and includes experience in service, construction and retail. He can be reached via e-mail at jott@haywardnet.com[2].

Source URL: https://www.poolspamarketing.com/trade/variable-speed-pump-economics/

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