variable speed vs const speed

8/8/2019 Variable Speed vs Const Speed

http://slidepdf.com/reader/full/variable-speed-vs-const-speed 1/6

PART 1: DEBUNKING THE MYTHS OF VARIABLESPEED

PRESSURE BOOSTING

Maybe “myth” isn’t the best word to describe the ongoingargument between those who advocate constant-speed andthose who support variable-speed pressure control systems. After all, variable-speed domestic water boosters are only now reaching adolescence as an accepted form of pressure controlin the plumbing market. Why all the misunderstanding? Doesthe answer lie in the desire to resist change? Is it due to the lack

of information available? Is there a left-wing conspiracy against variable-frequency drives (VFDs)? (Maybe I am being a littleparanoid!)

I asked some of these same questions (not the conspiracy-theory thing) while preparing for my presentation on vari-able-speed boosting for the 2006 ASPE Convention in Tampa,Florida. When properly evaluated, a variable-speed controlledpackaged booster system can be very cost-e ective if you look at some basic assumptions:• 80 percent of the time, a typical booster is at 20 percent

capacity or less.

• Building load demand is critically important to e ectivepower reduction.

• On the pump curve, both ow and head are signicant toe ective pressure control.

• Bladder tanks don’t store enough water to produce any sig-nicant energy savings.

• Variable speed reduces both sides of the electrical formula, yielding greater power reduction.

CONSTANT SPEED

Constant-speed systems are relatively straightforwardand simple designs involving the use of a constant-speedpump (the pressure source) with a pressure-regulating valve (PRV) downstream (the regulating device). As the

word implies, the pump never changes speed because itis powered by a starter. A starter has two positions: openand closed. erefore, a constant-speed booster systemessentially has two speeds: on and o . is concept has worked very well for many years due to its simple design, which is very common in older plumbing systems. In thepast, the need to have some type of pressure system wasof primary importance, whereas the integration of thesystem within the plumbing and piping design took abackseat to pragmatism. Nowadays, you need the prac-ticality of a constant-speed system with the energy e-ciency of a system that is not constantly running simply

to have water at the end of a xture when it is opened. In oth words, the engineer has had to match the customers’ demafor domestic water with the realities of the system load prole we match that load prole more closely, only supplying as mupower as needed for a particular moment, we have the oppornity to generate substantial energy savings.

THE 80/20 RULE

Let’s face it: All piping systems are di erent, with numeroand varied applications. A hospital has a di erent load pro

than that of an apartment building. It’s very dicult to

single, across-the-board constant-speed solution (which nereally adapts to the load prole) to piping systems that use wadi erently. e goal is to match the load prole more precisethereby reducing the energy required to run the system wh water usage is low. As a rule of thumb, I created what I call 80/20 rule for boosters (see Figure 1a). is rule states that (average) 80 percent of the time the booster system is operatiat 20 percent capacity or less. Based on this rule, a constaspeed system is not operating eciently during 80 percentits operational time, since it is not using much less power whrunning at 20 percent than when running at 100 percent capity. In fact, 80 percent of the time it is simply maintaining pippressure. Ideally, we should use the reality of the 80/20 ruleour favor as an opportunity for power savings.

THE PUMP CURVE

If we reduce the speed of the motor, we take advantagethe pump and fan Anity Laws, which state that powerreduced as the cube of the speed (see Figure 1b). is is imptant because on a pump curve, two dynamics are at play fr

Variable Speed V ersusConstant Speed

Figure 1a The 80/20 rule

14 Plumbing Systems & Design MARCH/APRIL 2007 PSDMAGAZINE

by David Carrier (Part 1) and Ryan B. Stickney (Part 2)



a demand perspective; typically the ow and head are chang-ing at the same time. Look at a typical pump curve. As the ow increases, pressure capacity drops, and vice versa. In a variable-speed application, as the ow decreases and the potential pumppressure increases, the drive is reducing that speed to match aconstant discharge pressure by slowing the pump down. As thepump slows, the Anity Laws tell us that the energy consumedis being reduced by a factor of eight.

In a constant-speed system, this does not happen becauthe pump does not change speed. Instead, the PRV becoma throttle that reduces potential head pressure by adding frtion and resistance through the closure of the regulator. Sinthis overpressure condition is a reality in the constant-spe world, manufacturers over the years have sought to take advtage of this “free” pressure by adding storage tanks in the hothat energy consumption could be mitigated by simply shutto the power to the system. It is a good idea, but unfortunat

this application collides headlong into the realities

Boyle’s Law, which states that the volume of a conngas at a xed temperature is inversely proportionathe pressure exerted on the gas.

THE BLADDER TANK

e bladder tank was added to the system to saenergy consumed by the constant-speed pump, wh was only running to maintain piping pressureinvolved using the excess pressure created by the puat shuto to charge a pressure tank, allowing waterbe stored so the system did not have to restart to msmall loads. In fact, the idea caught on so well thabecame an industry standard. Unfortunately, only hof the story was told. To charge a bladder tank, yneed a pre-charge equal to the system pressure bef you add even a drop of water (see Figure 1c).is means that the only advantage in storage is wh

the pump generates more pressure than it requires maintaining system pressure. After this pre-charlittle room is left in the tank for water. If you wantcalculate how much water a typical bladder tank hola simple rule of thumb is to gure about 10 percenthe tank volume. If your client buys a 200-gallon tanhe’ll have only about 20 gallons of water available shutdown periods. It can be more cost-e ective to rthe variable-speed system 80 percent of the time a

reduced speed than to run the constant-speed/bladdsystem 70 percent of the time (if that’s even plausibat full voltage. In cases of high demand, such as u valves, the demand is so quick that the pump will tycally start before the tank can even begin to satisfy rapid demand of the valve. I have seen bladder tanon variable-speed systems. As mentioned before, pressure must change for the tank to receive any stage. e VFD must increase its speed, thereby chaning its purpose to a variable-pressure system. Wiout increasing the speed and system pressure for purpose of charging the tank, the tank will nevercharged with any water.

THE SAVINGS

At the end of the day, the most important aspectthese decisions is the cost to operate the machineHow does the owner get operational eciency frthe system? is is where the engineer earns his pcheck. e key element to operational eciencyclearly stated on the energy bill. e owner is bilin kilowatts. What impacts the reduction of watts?electrical terms, this can be summarized by a sinelectrical law known as Ohm’s Law (see Figure 1Ohm’s Law states simply that volts multiplied by amequals watts.

Figure 1d Ohm’s Law drives the Affinity Laws?

Figure 1c Proper pre-charge for a bladder tank

Figure 1b The Affinity Laws

MARCH/APRIL 2007 Plumbing Systems & Design



In the case of the constant-speed pump, asthe ow is reduced, the amount of amperageconsumed is reduced, but the voltage remainsthe same. A typical squirrel-cage inductionmotor has a reduction in amperage draw from full load to zero load of about 60 per-cent, thereby reducing amperage at low ows,

which is only one variable of Ohm’s equation.If the same motor is attached to a variable-

speed pump, the amperage is reduced thesame; however, the voltage is reduced at anequal percentage to the reduction in speed.is means that a 460-volt motor running athalf speed while connected to a drive will bepulling only 230 volts at its rated amperagefor the pump capacity. In the variable-speedapplication, both variables of the Ohm’s Law equation are impacted, versus the constant-speed application in which voltage remainsconstant at all capacities. is is the primary reason that variable-speed pressure controlhas become an industry standard.

Constant-speed systems have served theindustry well for many years, but techno-logical enhancements increase just like yourclient’s power bill. With the pressure on theconsultant to optimize the system for energy eciency, the variable-speed system hasbecome an appealing option in the 21st Cen-tury economy. Perhaps the constant speedsystem will be relegated to the history books, just like those old myths of the past.

PART 2: GAINING ENERGY SAVINGS WITH

DOMESTIC WATER BOOSTER PUMPS

Variable-speed operation is now a standardin most of our specications, but how many of us can really explain its operation? What

are the dynamics? What is the operatingcost? How much energy can we save? Suchquestions are presented to the consultant onalmost every domestic water booster applica-tion.e proper response to these questionsis especially critical due to the increasingdesire of consultants, owners, and consum-ers to be environmentally responsible. efollowing will answer these questions andprovide a path to quantifying energy savings

for our clients.First things rst: Domestic water pumpingis a variable torque application. Pumps aremachines that have the benet of the Anity Laws.is allows us to reduce the motor torque,

which yields the highest energy savings. As mentioned in Part 1, the Anity Laws

state the following:• Law #1:e ow varies directly with the

speed or impeller size.

• Law #2:e head varies by the square of the speed or impeller size.

FEATURE: VARIABLE SPEED VERSUS CONSTANT SPEED

Figure 2a Single-speed pump curve

Figure 2b Multi-speed pump curve

Figure 2c Domestic water pump system curve




8 0 0 - 5 4 3 - 2 5 5

Copyright © Liberty Pumps, Inc. 2006 All rights reserved.

www. libertypumps .co

Engineered Pump System Now with QuickTreeTM Technolog

Grinder Systems

Solids-Handling Systems

Effluent Systems

Simplex

Duplex

Order a Liberty EPS and you’ll get assurance of the complete system arriv

to the job-site with factory matched co

ponents – pump, panel, basin, internal pip

guide rails, floats - all from one source.

All liberty EPS systems now feat

QuickTreeTM technology for quick and ea

access to the float switches!

Need it fast?

Specify a pre-designed system from Libe

Pre-designed simplex and duplex systeare available and ready to ship in 24 ho

Consult factory for more information on

pre-designed packages.

Complete Systems.

ONE Source.

Complete Systems.

ONE Source.

Circle 17 on your reader response card for product information.



• Law #3: e power varies by the cube of the speed orimpeller size.

• Law #4: e torque varies by the square of the speedor impeller size.

e pump Anity Laws work two ways: We may vary thespeed or vary the size of the pump impeller. We only get oneimpeller size, so herein lays the benet of varying the speedof the motor. Figure 2a is an example of a single-speedpump curve and Figure 2b is an example of a multi-speed

pump curve.e multi-speed curve gives us a precise look at the behavior of a pump when the speed is reduced.e system curve of a domestic water pump is at, run-

ning from the design point horizontally back to shuto (see Figure 2c). You should be concerned with three areasof the curve: the design point, the 60-hertz (Hz) shuto pressure, and the system curve (the system curve pres-sure is the same as the pump’s required boost). As the60-Hz or constant-speed curve rides toward shuto , itrises, increasing the pump’s capable pressure. e dif-ference between the pump’s capable pressure and thepump’s required boost determines the pump’s speed andits potential energy savings.

A minimum speed exists in this application. We mustmaintain a minimum pressure in a domestic water systemto maintain proper pressure at the top of the building.Since the speed varies as the square of the head, we arelimited in our ability to reduce it. e di erence betweenthe pump’s capable shuto and the pump’s required boostdetermines its minimum speed. Minimum speed is criti-cal in determining variable-speed operation and potentialenergy savings. In Figure 2d, the pump’s capable shuto is300 feet, and the pump’s required boost is 200 feet. esecond Anity Law is used to determine the minimumspeed. Following is the equation used to solve for variabletorque speed in the Figure 2d example:

3,500 revolutions per minute/(300-foot shuto /200-foot pump required boost)^(½) = Minimum speedus, the minimum speed is 2,860 rpm.e minimum speed will uctuate if there is a variance

in suction pressure. If the suction pressure increases, thepump works less, and vice versa. Remember: We sizeboosters based on the minimum city suction pressureavailable. is minimum city suction is not always real-istic. In many cases, the city guarantees a lower pressurethan actually exists.

Pump selection is critical to variable-speed operation.e curve in Figure 2d is relatively steep. Let’s compare

this steep curve to a

at curve (see Figure 2e). We apply the same formula using the second Anity Law:3,500 rpm/(240-foot shuto /200-foot pump required

boost)^(½)us, the minimum speed is 3,195 rpm. (Again, the

minimum speed will uctuate if there is a variance in suc-tion pressure. If the suction pressure increases, the pump works less, and vice versa.)

In comparison, the minimum speed is reduced as thecurve gets steeper. erefore, selecting a steep curve versus aat curve yields greater energy eciency. Witheither curve, your selection should be as far right on the

FEATURE: VARIABLE SPEED VERSUS CONSTANT SPEED

Figure 2e Flat curve

Figure 2d Steep curve

Figure 2f Determining energy cost for domestic water pumps




curve as possible, which ensures the greatest operating bandwidth. Decreased axial and radial loads on the bearings andseals are additional benets to operating at a lower speed.e next step in determining your operating cost is to deter-

mine the building’s load prole. e 80/20 rule is very helpful when determining the building’s load prole. e 80/20 ruleis not a substitute for an accurate load prole. It is simply theaverage of most system types. is rule does not apply to every application, and the only way to precisely quantify the operatingcost of a pump is to use a meter while it is operating. However,

we can give our clients a fairly close representation of operatingcost using this rule in many building types. We should be realis-tic when determining the building’s load prole. A typical loadprole would state: 80 percent of the time the ow is 20 percentof the design point; 10 percent of the time the ow is 50 percentof the design point; 5 percent of the time the ow is 80 percent of the design point; and 5 percent of the time the ow is 90 percent of the design point. An energy calculation must be performed duringeach stage of operation (see Figure 2f).

Once determining the load prole, you canbegin to quantify the operating cost. We begin with the minimum speed, so we look back to

our example in Figure 2d. Remember that therst Anity Law states that the speed directly changes the ow. To dene your operating bandwidth, subtract the minimum speed from therated speed. In the following example, the oper-ating bandwidth is 640 rpm.e minimum speedplus 20 percent satises the 80/20 rule; therefore,this system will operate at 2,980 rpm 80 percentof the time. Repeat this step for each portion of the load prole.• 3,500 rpm – 2,860 rpm = 640 rpm

• 640 rpm * 20 percent = 120 rpm

• 2,860 rpm minimum + 20 percent = 2,980 rpmNext, determine the variable torque horse-

power (HP), amps, and kilowatts (KW) for eachportion of the load prole. Change the hours of operation for each portion based on the percent-age of operation.

• HP (variable torque) = Rated HP * (Speed variable torqueRated speed)^3

• Amps = (746 * HP)/(1.732 * Volts * Eciency * Power facto

• KW = (Volts * Amps * Power factor * 1.732)/1,000

• KW * Hours (8,760 hours/year) * Energy rate (Average $0.0per KWh) = Operating cost

We have an obligation to provide our clients with the most enronmentally ecient systems available.is will become increingly important as energy costs rise (24 percent in Illinois as

January 1) and resources are depleted. By accurately determinbuilding load proles and selecting energy-ecient pumps, are well on our way to providing our clients with environmentaecient domestic water pump systems.

MARCH/APRIL 2007 Plumbing Systems & Design

DAVID CARRIER is the executivevice president of Delta P. SystemsInc. in Ormond Beach, Florida, apackaged systems manufacturer and

pioneer in the use of variable-speedcontrols in the plumbing market.

RYAN B. STICKNEY is thegeneral manager of Mechanical

Room Inc. in Chicago. Ryanspecializes in plumbing and HVAC

pumping systems and variable-frequency drives. He is also the

author of engineering software thatassists engineer with domestic water

pumping operating costs whenusing variable-frequency drives.

For more information or to comment on this article,e-mail [email protected].

Available with:

• Manways and Extensions

• Traffic Bearing Covers

• Superior Corrosion Protection Systems

with 30 year limited warranty• Rugged Acid Resistant Linings

• Double-Wall Construction

• Stainless Steel Construction

Features:

• Superior Quality and Workmanship

• Easy Installation and Maintenance

• High Performance and Reliability

814-443-6800 • FAX 444-8662

[email protected]

High Quality - Large Capacity

Multi-Compartmented Interceptors

• Sized to comply with all National and

Municipal Plumbing Codes.

• Designed to meet EPA and municipal

sewer pretreatment discharge limits for

fats, oils, grease, and solids.

Two Popular Styles for Sanitary

Sewer Protection:

• Passive Grease Interceptors remove large

quantities of fats, oils, and grease (FOG),

and solids discharged from food servicefacilities and large commercial and

institutional kitchens.

• Sand/Oil Interceptors separate sand, oil,

and grease (hydrocarbons and other

petroleum products) from automotive

repair, car washes, and other petroleum

and industrial facilities.

STOPOil, Grease & Solids

at the Source

www.lowe-engineering.com

Circle 19 on your reader response card for product information.

variable speed vs const speed

Documents