Last week we used the Pump Affinity Laws to determine what size the impeller would be trimmed to in an oversized pump. Reducing the speed of the pump has the same effect on the flow and head as trimming the impeller.

Lets look at the example again. The engineer specified an ITT Bell and Gossett VSX- VSC double suction pump in a closed chilled water system. The capacity is 3500 GPM at 140 ft head. The pump selected is shown below.

The balancing contractor reads the pump out after a proportional balance, and finds the operation point to be 4300 GPM at 120 ft. This means the pump is oversized.

As we discovered last week, the impeller could be trimmed to 10.75” to correct the problem. If this pump is operating on a variable speed drive, the speed could be reduced to design flow rate. What speed would we set as the maximum? To determine the correct speed, use the pump affinity law.

The design RPM on the curve is 1780. Check the nameplate of the motor to verify the actual speed of the motor. Each motor manufacturer may have a slightly different speed and usually it does not make much difference, but it is worth checking. If it is different, you can call your local B&G representative for a curve at the specific RPM of the motor. Let’s assume the motor nameplate is 1780 RPM.

Just solve the problem. (GPM1/GPM2) = (RPM1/RPM2) = (Hertz1/Hertz2)

So GPM1 is 3500; GPM2 is 4300; RPM1 is unknown; RPM2 is 1780.

The answer is 1448 RPM or about 1450 RPM. Since hertz and speed are directly proportional, the 1450 RPM speed will result from dropping the drive to about 50 hertz. The resulting curve looks like this:

If you would like to download the electronic system syzer go to http://www.bellgossett.com/BG-SystemSyzer.asp.

Thank you for using products sold by R. L. Deppmann Company in Michigan and Ohio!

Disclaimer: R. L. Deppmann and it’s affiliates can not be held liable for issues caused by use of the information on this page. While the information comes from many years of experience and can be a valuable tool, it may not take into account special circumstances in your system and we therefore can not take responsibility for actions that result from this information. Please feel free to contact us if you do have any questions.

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The Pump Affinity Laws are used to determine the system curve. If we know the point of operation on a pump curve, we can determine the effect of trimming impellers or changing speeds.

Let’s look at an example. The engineer specified an ITT Bell and Gossett VSX- VSC double suction pump in a closed chilled water system. The capacity is 3500 GPM at 140 ft head. They selected the B&G VSC because of the great benefits to the owner. Benefits that include silicon carbide unitized seals, ease of service, ANSI/OSHA coupler guard, and a good 15 other reasons. The pump selected is shown below.

The balancing contractor reads the pump out after a proportional balance and finds the operation point to be 4300 GPM at 120 ft. This means the pump is oversized. The over sizing may be a result of pipe changes during the construction, estimates of pressure drop through equipment, changes in equipment from the basis of design, or safety factors.

The balance contractor is asked what the impeller should be trimmed to. The balance contractor takes scale 5 of the System Syzer and sets 4300 GPM against 120 feet. Using the B&G system syzer the field tech continues to plot the system curve below. The intersection of the system curve and 3500 GPM is at 80 feet. The impeller could be trimmed to 10.75”.

If you would like to download the electronic system syzer go to http://www.bellgossett.com/BG-SystemSyzer.asp.

Thank you for using products sold by R. L. Deppmann Company in Michigan and Ohio!

Disclaimer: R. L. Deppmann and it’s affiliates can not be held liable for issues caused by use of the information on this page. While the information comes from many years of experience and can be a valuable tool, it may not take into account special circumstances in your system and we therefore can not take responsibility for actions that result from this information. Please feel free to contact us if you do have any questions.

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Hydronic heating system flow rates and pressure drops are related through a series of AFFINITY LAWS. Let’s examine the laws and how to use them in practical day to day applications.

This first law tells us if we have a pump operating at design conditions, in a closed hydronic system, and it is flowing “X” GPM; if we cut the speed in half we will cut the flow in half. If we trim the impeller, then the flow will drop in a direct relationship.

The second Affinity Law is a bit more complicated. It says, “The change in pump head or pressure drop in the system varies as the square of the change in GPM flow rate. If the system pump is flowing “X” GPM at “Y” feet of head and we want to double the flow rate; the pump will have to produce 4 times the head. Conversely, if we want to drop the flow rate in half it only takes ¼ of the pump head. Of course, all of this assumes a steady state condition, meaning the control valves are all at design condition or not changing as the flow changes.

 
Scale 5 on the B&G System Syzer can be a handy tool for using the second Affinity Law. Let’s take a look at an example. Assume you have a heat exchanger with a capacity of 100 GPM at a pressure drop of 20 feet. Maybe that 100 GPM was based on an original design of 20°ΔT and you want to see what the pressure drop would be if you went to 40°ΔT with the same BTUH output. We know that BTUH = GPM x ΔT x 500 for water, so if we change from 20°ΔT to 40°ΔT then the GPM goes from 100 GPM to 50 GPM.

Now let’s find the new pressure drop using scale 5 of the System Syzer. We could just use the formula above and a calculator, but let’s find a quick answer. Take scale 5 and align 100 GPM in the white window with 20 feet in the blue window. Now look at 50 GPM in the white window. Without moving the wheel, we see 5 feet of pressure drop. We obtain a quick answer using the second Affinity Law and the ITT Bell and Gossett System Syzer.

Next week we’ll use this tool for a field pumping problem.

If you would like to download the electronic system syzer go to http://www.bellgossett.com/BG-SystemSyzer.asp.

Thank you for using products sold by R. L. Deppmann Company in Michigan and Ohio!

Disclaimer: R. L. Deppmann and it’s affiliates can not be held liable for issues caused by use of the information on this page. While the information comes from many years of experience and can be a valuable tool, it may not take into account special circumstances in your system and we therefore can not take responsibility for actions that result from this information. Please feel free to contact us if you do have any questions.

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     The ITT Bell and Gossett system syzer has a scale 5 which may be used to determine pressure drop in a hydronic system when the Cv (called C sub V) of a valve or device is known. The Cv is the water flow, in gallons per minute (GPM), which causes a 1 PSI pressure drop.

     If a hydronic control valve has a Cv of 20, then the valve will have a pressure drop of 1 PSI, when 20 GPM flows through it. So the units of Cv are GPM. Let’s use scale 5 of the B&G System Syzer® to find the flow rate if we have 5 PSI pressure drop across this valve, which has a Cv of 20. Remember, in field measurements, that the Cv was determined with the device full open. Any field measurements, in which you use Cv, should have full possible flow, so drive the controls full open when measuring.

     Rotate the scale so that the Cv line, in the blue area, is lined up with 20 in the white section. Now you can look at any flow rate in the white area and find the pressure drop, in feet, above it and PSI below it. In our example, with 5 PSIG pressure drop across the valve, we see the flow rate will be about 47 GPM.

     Conversely, if you know the flow and head of a device and you are asked about the Cv, you may use scale 5 to find it. Let’s assume you know a coil has a pressure drop of 6 feet at 200 GPM. What is the Cv of the coil? You guessed it! Take scale 5 and rotate until 200 GPM in the white section lines up with 6 feet above it in the blue area. Make sure you don’t mix up feet and pounds. Now read the Cv in the white area. It is about 122. The electronic version of the System Syzer will give you exact numbers.

Next week we will use scale 5 of the system syzer in a pumping application…

If you would like to download the electronic system syzer go to http://www.bellgossett.com/BG-SystemSyzer.asp.

Thank you for using products sold by R. L. Deppmann Company in Michigan and Ohio!

Disclaimer: R. L. Deppmann and it’s affiliates can not be held liable for issues caused by use of the information on this page. While the information comes from many years of experience and can be a valuable tool, it may not take into account special circumstances in your system and we therefore can not take responsibility for actions that result from this information. Please feel free to contact us if you do have any questions.

Archives - Click here for Past Articles

     In the last two MMM articles we used examples of pipe selection for water and other fluids. Using the electronic version of the ITT Bell and Gossett system syzer we found our example of 800 GPM in 6” pipe produced a pressure drop of 4.03 feet/100 feet for water and 5.04 feet/100 feet for 50% ethylene glycol/water at 40° F.

     We can use this valuable tool for another purpose. When using a fluid other than water in hydronic systems it is, often times, necessary to correct a water pressure drop of a piece of equipment to the pressure drop for the fluid.

     The ITT Bell and Gossett manual TEH-176 Hydronic Antifreeze Design describes some approximate correction factors for 50% ethylene glycol. The manual indicates the correction factor for 40°F – 50% ethylene glycol is 1.22 in a heat exchanger. What happens if you need a correction factor for other fluids? The system syzer can help!

     Example: Let’s assume our 800 GPM fluid is now 40% propylene glycol/water at 25°F. Using the methods described in the last MMM of 1-11-10 we correct for the fluid type and find the pressure drop is now 5.96 feet/100 feet. Since the number was 4.03 for water, we can take 5.96 ÷ 4.03 = 1.48 and use it for our correction factor. This is an approximate correction since it changes with the Reynolds number and velocity throughout the system.

     An engineer may find a situation where the system pressure drop is based on water and he or she is looking for a quick correction factor to change to another fluid. This method gives a quick multiplier to approximate the pressure drop change.

Next week we will look at the Cv portion of the system syzer.
Click here to view or download a copy of the ITT Bell and Gossett TEH-176 manual.

If you would like to download the electronic system syzer go to http://www.bellgossett.com/BG-SystemSyzer.asp.

Thank you for using products sold by R. L. Deppmann Company in Michigan and Ohio!

Disclaimer: R. L. Deppmann and it’s affiliates can not be held liable for issues caused by use of the information on this page. While the information comes from many years of experience and can be a valuable tool, it may not take into account special circumstances in your system and we therefore can not take responsibility for actions that result from this information. Please feel free to contact us if you do have any questions.

Archives - Click here for Past Articles

Last week we used the ITT Bell and Gossett system syzer to select a pipe size for 800 GPM. We used the wheel and the electronic version. Today, let’s expand the selection to include a fluid other than water. To use fluids other than water you must use the electronic version. There were two solutions from last week’s example using water:

Now let’s change the fluid in our example from water to 50% Ethylene Glycol at 40° F. Start by clicking on the “fluid” at the top of the page and remove the 60° F water selection to allow changes. Select ethylene glycol and enter the 40° F and 50% options. Notice the viscosity jumps to 6.65 and the specific gravity is 1.07. We can expect an increase in friction loss with this new fluid.

Select “OK” and we see 6” pipe with a pressure drop of 5.04 feet/100 feet and 8” pipe at 1.33 feet/100 feet. This may make us decide to change from the 6” pipe to the 8” pipe to keep the pump horsepower lower.

Next week we will use this information for other correction factors.

If you would like to download the electronic system syzer go to http://www.bellgossett.com/BG-SystemSyzer.asp.

Thank you for using products sold by R. L. Deppmann Company in Michigan and Ohio!

Disclaimer: R. L. Deppmann and it’s affiliates can not be held liable for issues caused by use of the information on this page. While the information comes from many years of experience and can be a valuable tool, it may not take into account special circumstances in your system and we therefore can not take responsibility for actions that result from this information. Please feel free to contact us if you do have any questions.

Archives - Click here for Past Articles

     Many engineers and contractors in the HVAC industry are familiar with the B&G red-white-blue system syzer wheel and use it often for scale 2 pipe sizing. Over the next few weeks the MMM articles will review the use of the hand held wheel and the electronic version. Today we will review Scale 2 and Scale 3.

     The wheel: To use the Scale 2 pipe sizing just rotate the wheel until the flow rate in GPM shows up in the white pie shaped area. Now rotate the wheel slightly until the arrow on the top of the “pie is aligned with a pipe size. The top portion of the “pie” gives you the pressure drop for per 100 feet of (TEL) total equivalent pipe using water as the liquid. The “pie” allows a selection range of approximately 1 to 4 pressure drop per 100 feet of TEL. Scale 3 gives the FPS (feet per second) velocity.

     Example: Select a pipe size for 800 GPM. Solution: Rotate the wheel until 800 GPM is in the “pie”. There are two choices for the pipe size. 6” schedule 40 steel pipe will have 4 feet/100 feet pressure drop and 8” steel pipe will have 1.05 feet/100 feet pressure drop. If we select the 6” pipe scale 3 shows the velocity at just under 9 FPS.

     On the electronic version; click on the flow/pressure drop relationship button; then enter the flow rate; now choose pipe sizing to get the pressure drop between 1 and 4 feet/100 feet. The electronic version also gives you the Reynolds number and friction factor as well as the velocity.

Next week we will use the electronic System Syzer® and change the fluid type.

If you would like to download the electronic system syzer go to http://www.bellgossett.com/BG-SystemSyzer.asp.

Thank you for using products sold by R. L. Deppmann Company in Michigan and Ohio!

Disclaimer: R. L. Deppmann and it’s affiliates can not be held liable for issues caused by use of the information on this page. While the information comes from many years of experience and can be a valuable tool, it may not take into account special circumstances in your system and we therefore can not take responsibility for actions that result from this information. Please feel free to contact us if you do have any questions.

Archives - Click here for Past Articles

Disclaimer: R. L. Deppmann and it’s affiliates can not be held liable for issues caused by use of the information on this page. While the information comes from many years of experience and can be a valuable tool, it may not take into account special circumstances in your system and we therefore can not take responsibility for actions that result from this information. Please feel free to contact us if you do have any questions.

Archives - Click here for Past Articles