It is well documented that varying the speed of a pump to meet part-load conditions is a great way to save energy. The savings occur due to the Third Pump Affinity Law, which states that the power consumption of a pump varies with the CUBE of the speed reduction. The actual reduction achieved in an installed system may be somewhat less, though still VERY significant (to determine actual savings requires a knowledge of the application's "system curve," a topic covered in one of our FH Institute hydronic courses). Note 1
While speed reduction is a great thing for energy savings, TOO MUCH speed reduction results in rapid failure of the pump's mechanical seal. To understand exactly why, we must understand something called hydro-dynamic forces. Let's start by reviewing the construction of a typical mechanical seal.
As shown in this figure, a seal consists of a stationary element, the seat, which is pressed into a machined cavity in the pump casting and held by an "O" ring, and a rotating element, which is pressed onto the pump shaft,Note2 and held in place by an elastomer drive ring. Note the face of the rotating element is normally a softer material than the face of the seat, and that both faces are manufactured to be flat to within very fine tolerances.
In operation, several forces act to maintain a very small gap between the faces of the rotating element and the stationary element. The gap fills with system fluid (usually water or glycol in HVAC systems), which lubricates the seal faces, preventing overheating and failure, which would occur with direct face-to-face contact and rotation. Yet the gap is small enough that any leakage should be undetectable. The forces that create this gap include:
- A force created by pressure in the pump acting on the back (right) side of the rotating element, somewhat counterbalanced by the same pressure acting on the smaller surface "front" surface of the rotating element. The net effect is to push the rotating element toward the seat.
- A pressure force in the gap created by pressure in the casing acting on the inner rim of the rotating element as it "attempts to escape" to atmospheric pressure. It acts to separate the seal surfaces.
- Spring force. The spring provides resilience in the seal system and acts to maintain the gap between the fixed and rotating elements as the seal faces erode. It pushes the rotating element toward the seat.
- A "hydro-dynamic" force, which forms when the shaft rotates, acts to push the rotating element away from the seat. Though hydro-dynamic forces are not completely understood, they are thought to consist of: (a) Centrifugal force, caused by fluid that spins outward between seal faces during rotation, and (b) A wave-like action that forms in the fluid between the seal faces. This wave is thought to be created, during rotation, by minor surface irregularities in the "flatness" of the faces. This wave action creates a force that separates the faces.
Under normal operation, the seal is designed so that the forces resolve to maintain just the right amount of gap for the application – enough that seal face lubrication is maintained, but small enough to prevent detectable leakage.
Though we don't completely understand the mechanism of hydro-dynamic forces, we know that they increase with increasing shaft speed and decrease with decreasing shaft speed.
Q: So, what causes the seal to wear rapidly of speed is too low?
A: All seals wear OVER TIME. But when shaft speed slows excessively, the hydro-dynamic forces decrease to the extent that the seal faces either actually touch, or become so close that excess heat is generated. Either event causes rapid seal wear.
Q: So, what is the solution?
A: The answer is to maintain a minimum RPM that is high enough to create hydro-dynamic forces sufficient to maintain a "healthy" seal gap. As you might imagine, every pump, every seal, every application is different, so we are left with rules of thumb that have proven to be relatively "safe."
Taco Inc.'s current recommendations are as follow:
Motor/Pump Nominal RPM Min. Allowable Speed Min. Drive HZ Note 3
1200 500 25
1800 450-600 15-20
Note that many pump applications serve systems with significant static head, or systems where a pressure set-point must be maintained. In most of these systems, a slower speed than the recommended minimum may never occur. The pumps need to rotate at a higher speed than the minimums stated in order to meet the pressure requirements. But in other applications, (a variable speed boiler secondary pump is a good example), where no minimum head requirement exists, pumps COULD be called upon to operate at very low speeds. To be safe, in ALL applications, the recommended solution is to program a minimum speed into the variable frequency drive that meets the values above.
Q: Are there other considerations with variable speed pumps?
A: Maybe so. It stands to reason that, over time, the seal faces will likely operate with a smaller gap than in a fixed-speed application. It seems logical that finer particles of scale and iron oxide would, then, cause grooves in the seal, particles which may not be a problem with a bit wider gap. Though we have read of no specific studies, it does make extra sense to strive for system cleanliness. It would be beneficial, in the interest of extending seal life, to install an air/dirt separator (similar to the Taco 4900) plus a bypass filter in the system.
In summary, it is important to understand the role of hydro-dynamic forces in pump application and that they affect the minimum recommended pump speed.
Note 1: See the FH Institute page for upcoming and correspondence courses.
Note 2: Most commercial pumps actually are equipped with a non-corrosive shaft sleeve that is pressed over the actual pump shaft. The rotating element is pressed onto this sleeve.
Note 3: These recommendations are based on pump considerations. Motor & drive limitations MAY be higher. Check with manufacturers of those devices.
THIS MONTH'S FEATURED PRODUCT - Unilux Boilers
Unilux Boilers is home to the world's original 5-pass forced draft bent tube design. With over thirty years of manufacturing and operational experience, Unilux has products for just about every application in the boiler industry.
• The Unilux steel bent water tube boiler design was patented in 1981.
• The Unilux Boiler is the second oldest bent tube boiler available on the market today.
• Unilux became American owned in year 2000.
• In 2004, corporate expansion increased manufacturing to a new state of the art 67000 sq. ft. production US plant.
Design features include natural internal circulation, service friendly design, and thermal shock proof with a standard twenty-year warranty, fast recovery rate, and a four (4) sided water wall furnace.
• 81% Efficiency Low Pressure Steam Boilers
• 81% Efficiency High Pressure Steam Boilers
• 85% Efficiency 5-pass Hot Water Boilers
Unlike other bent tube boilers, Unilux has a large volume furnace of tangent tube design, allowing for a 4-sided water wall heat release and a positive flow through all boiler tubes, as each is exposed to direct radiant heat. Therefore, the radiant heat from the burner flame is not absorbed by refractory and lost to ambient; it is transferred directly to the tube side fluid.
The volumetric furnace heat release of the Unilux boiler is no more than 80,000 BTU/HR/CU.FT. depending on boiler size. Equally or more importantly, the heat release per sq. ft. of heating surface in the Unilux design is never greater than 45,000 BTU/HR/SQ.FT. This value represents the lowest furnace heat release of all steel boiler designs in the market today.
This inherently provides the greatest protection against metal fatigue as well as being an integral design feature, which helps the Unilux (5) pass design achieve the highest available operating efficiencies.
The Unilux 5 pass boiler is a true forced draft design encased in a heavy duty insulated steel housing. Industrial grade housing panels are well insulated with 3" of high temperature fiberwool. The entire casing, when completed allows for up to +5" water column pressure. The housing is further insulated with a steel jacket employing a 2" air insulator space between the inner and outer panels for maximum insulating effect. The front and rear boiler walls are made with our exclusive 3-tier process. First, a 4" layer of insulating refractory is poured, second a 1" layer of high temperature fiberboard installed, and third, a 4" layer of 2700 deg. F refractory is poured. All this is tied together with stainless steel anchors closely spaced and welded to the outer steel housing. Operating radiant losses are calculated at ¼ of 1% of maximum input. Larger boilers (model 2000 and up) have industrial grade 12" and 10" walls for ultimate dependability.
Unilux features large volume down comers (one for water designs and two for steam designs) totally isolated from the fireside to provide maximum positive circulation within the pressure vessel. Water flow resistance is minimal, as such; external circulating devices are not required with the Unilux design.
Both the fireside and waterside are easily accessed for inspection and maintenance though strategically located ports on the assembly.
Unilux boiler tubes are no less than 1 ½" SA178A, electric resistance, welded carbon steel high quality construction. Tubes are generally fitted with a precision machined, tapered ferrule that is driven into the machined boiler drums. There is no welding or rolling required. Tube replacement, if required is easily achieved without major disassembly of the boiler.
The Unilux UL/FE boiler is another Unilux first. We are proud to have tackled the job of producing the world's first and only line of Underwriters Laboratories Listed and Labeled field erect boiler assemblies. The specifying of a Unilux UL/FE provides the highest available safety and accountability procedure for the field erect market.
• High thermal efficiencies due to the high number of Gas Passes (5)
• Field erect design for getting into tight spaces
• Modulating turndown ratio from 4 to 8:1 depending on the type of burner selected
• Firing on Natural Gas or # 2 Oil, exceeding environmental regulations
• Flexible tube design, eliminating the possibility of thermal shock
• Large Steam drum, capable of handling large load swings
• Excellent steam quality, due to better internal circulation
• Natural boiler circulation, eliminating the need of an extra pump. No circulating pump required.
• No tube welding required, making repairs easy and having the lowest repair cost. Tubes are ferruled fitted
• Low electrical, maintenance and fuel operating cost. Request a Free Evaluation
• Lower Installation Cost - Low replacement parts cost, as most parts are available from any supplier
• Most comprehensive Warranty's available, including Thermal Shock (25 Years) & refractory warranty of 5 yrs.
If you'd like more information on Unilux or any of our other products, or would like to schedule a lunch and learn at your company, please contact Andrew Fleck (email@example.com) or at 414-358-2646.
When an old cast iron boiler fails, the decision arises as to whether a condensing boiler would be worth the added investment. Most older systems designed to operate with cast iron boilers were designed with water temperatures of 160 degrees F to 200 degrees F.
Perhaps a quick and dirty review of condensing boiler theory is in order. Condensing boilers achieve their rather remarkable efficiencies by operating at low temperatures (return water temperatures at less than 120 degrees F or so, depending upon manufacturer and model).
The low return water temperature works to remove a lot more heat from the hot combustion gasses, dropping them to temperatures below their dew point. The gasses contain a good percentage of water vapor, being that the fuel (natural gas or propane, typically) is a hydrocarbon, in which water vapor is formed during combustion, as the hydrogen molecules combine with oxygen. When the combustion gasses are lowered to the dew point temperature, the water vapor begins to condense into liquid water. And when that happens, an extra (roughly) 1,000 BTU per pound of condensed water is released to the hydronic heating system. The result is that condensing boilers operate with much higher efficiencies that non-condensing boiler.
Next, let's focus on the system, the one that was designed to operate at 180 degrees. Conventional wisdom is that you cannot reap the benefits of a condensing boiler in such a system, as the return water temperature will never be low enough to force condensing to happen. Well, maybe. In fact, I have been involved in any number of such conversions and all were successful to some degree, in achieving condensing. How can this be?
Let's look at some factors. First, realize that newer boilers almost always include hot water reset as a standard or standard option, so it is a simple matter to set up a varying hot water temperature schedule. Mild day...low water temperature. Cold day...high temperature. Most days are mild days, and depending on the system, possibly mild enough to allow condensing operation for a good number of hours annually.
Next would be oversizing. Many old boilers heating systems were very much oversized, so that a room needing 12' of fin tube radiation might have 16' or 18' or...you get the picture. So maybe 180-degree water really is not required.
Lastly, the type of building and occupancy is a factor. The most successful condensing retrofit I have been part of was an office space with high internal loads. Relatively high occupancy, high lighting levels and each person had a computer feeding multiple servers. When occupied, we found that the building could heat successfully with 130-degree water on the coldest days (in Wisconsin!) and lower temperatures in mild conditions. Being well insulated, the building only lost a few degrees at night and on weekends, no more than normal night set back.
Certainly, there are many detailed considerations in the design of a condensing retrofit. For example, it may be necessary to add a system volume tank due to the small mass of many of today's condensing boilers. And you certainly don't EVER want to cause extensive condensing in a non-condensing boiler. I hope to write a course detailing some of these factors in the coming year. But, the key element of this post is that one should never assume that a condensing boiler could never pay for itself when installed in a system designed for non-condensing temperatures. A survey, including a good load and installed equipment analysis is the place to start. You might be surprised!
While feeling a person's forehead to test their temperature might be a good indication of that person's health, the "feel test" is not a good indication for the health of a pump motor, or any motor.
While motor manufacturers design their motor windings to certain operating temperature limits (to ensure long winding insulation life), there are many factors that contribute to the actual surface temperature of the motor housing. Factors include:
- Type of motor enclosure (ODP, TEFC, etc.)
- Whether surface is smooth or ribbed
- How fully the motor is loaded vs. the motor rating
- The motor efficiency rating (Note: Premium efficiency motors might exhibit a higher surface temperature than a standard efficiency motor, due to a smaller internal fan designed to consume less parasitic power.)
Depending on these and other factors, the surface temperature of a properly operating industrial motor may be 180-212 degrees F, so that even a quick touch test could result in a burn. An overloaded motor may run hotter, but who among us could leave our hand on the motor surface long enough to tell if the motor is hotter than 180-212 degrees?
Obviously, if a motor trips out on overload, smokes, or smells acrid (from "burning" insulation), these symptoms should not be ignored. A multi-meter can be used to see if the motor is drawing greater than name plate amps (improper flow balancing is the most common cause of high amp draws in pump applications ) and a megohmmeter can test insulation integrity. But the touch test does not provide a lot of useful information, may result in a burn, and give erroneous information about the health of the motor.
Well, it may or may not be a big deal. There are a number of issues to consider. We will not consider the issue of fluid compatibility with materials. That will likely not be an issue but best to check the compatibility of things like valve elastomers. The things you do want to worry about are heat transfer and pump performance.
This blog is mainly about pump performance, but let's touch on heat transfer, too. But first, let's go back a step and talk about the properties of glycol solutions that matter. As opposed to water, glycol solutions have different viscosities (resistance to flow), specific gravities, specific heats and thermal conductivities than water. The effects on performance are cumulative for the various properties (they add up). They depend on the concentration (30%-40%-50%?) and they depend on the temperature. As this is but a humble blog, and not a full-blown presentation, let's avoid the murky details and go to the bottom line...
Heat transfer: I think I could make a safe statement that the heat transfer will be less effective after adding glycol. How much will vary on %, temperatures and approach temperatures. But you will likely lose capacity and the only way to accurately determine how much is to contact the equipment manufacturer of your coils, chillers, boilers, fan coils, etc. For sure, you will have to pump MORE glycol to get the same performance. Note that depending upon concentration, you WILL need about 5-10% more flow to achieve the same BTUH capacity at the design delta T of the system.
Pump performance: Again, this is but a blog but here is are a couple things to remember. 1) Higher percentage, more effect, 2) Temperature affects pump performance---cold temperatures (40 degrees F, for example), have much more effect than 190 degrees F). 3) Propylene glycol has greater effect than ethylene and 4) Glycol affects small pumps (<100 GPM) more than large pumps.
This is all covered in more detail in our course HYD-120 but here is an example: Let's say that you want to go from water to a 40% propylene glycol solution in a system making 40-deg F chilled water. First of all, the friction resistance of the system would increase by about 40%. So right away, you are pretty far behind the eight ball! Now, realize that in addition to affecting the system, these parameters affect the performance of the pump itself. Per the factors presented in HYD-120, your existing pump will be about 10% short of its original capacity The head will be about 10% short...uh-oh...this temperature and concentration will result in about 40% more pressure drop than water at the design flow rate and almost 50% more to hit the new flow rate. On top of all that, your power required will be about 5% higher for this viscous glycol than water.
Net effect, very approximately, you need:
1.1 (more flow required) X 1.1 (pump flow derate correction X 1.05 pump power increase X 1.4 friction loss correction increase) = About 1.78 times as much power to make it all happen!
So, is the moral of the story to figure more to double the power and select bigger pumps for all glycol solutions? Nope. For a 500 GPM system utilizing 30%, 190-deg F ethylene glycol, the corrections would be basically insignificant!
So, the moral of the story is 1) dig into it, do the math and do not assume, 2) be very wary of glycol type and percentage, pump size and especially cold glycol! Sorry---it simply isn't all that simple!
Generally, a flex-coupled pump will be aligned (that is, the motor and pump shafts will be aligned) prior to shipment, either at the factory or at the shop of the pump distributor. This often leads owners and installing contractors to think that they do not need to check alignment during the installation phase. This is a myth.
When a pump is shipped on a truck to the construction site, it undergoes a great deal of shock and vibration. One study I read yesterday measured peak G-forces at 10G's. Said another way, a pump weighing 800# undergoes forces of 8,000#, probably several times during the shipment and lesser forces constantly, as the truck starts, stops, turns and hits bumps in the road. In addition, a good deal of vibration exists in transit.
Consider that it really is friction that holds the pump and motor in place. Yes, there are hold down bolts, but the bolts really serve as devices to increase the force between the pump base and the pump/motor feet.
The combination of forces and vibration overcome friction and often cause the pump and motor to shift a bit during shipment. The amount of "shift" will often not be enough to be noticeable to the naked eye but will be enough to exceed the misalignment limits of the coupling.
The results are reduced coupling life and very often, there will be a noticeable vibration when the pump is operated. More severe misalignment may eventually result in early bearing failure.
Therefore, it is always best if flexibly-coupled pumps be aligned as part of the installation procedure. For a consulting engineer, this means a spec item that calls for field alignment as part of the commissioning process. For a contractor, this means to provide an alignment as an element of quality control, even if the spec does not call for it. For the owner, it means asking for field alignment and an associated report for any flex-coupled pump purchased. The alignment may be performed by the contractor or the pump supplier.
In the HVAC industry, it was once common and still is in some situations, with certain coupling types, a matter of a straight-edge alignment to meet the pump and coupler manufacturer' requirements. The trend, however, is for increased use of optical/laser alignments, which are generally more accurate, at least in the hands of experienced technicians.
So, it's not that one should not trust the factory or distributor alignment. It's simply a fact that things will often shift during transit. To guard against early coupling and even bearing failure, a field alignment really IS important.
Over the next few blogs, I will attempt to address some myths that exist regarding centrifugal pumps. Today we will talk about reverse rotation. Reverse rotation may exist for either of two common reasons:
1) The most common reason is that when a three-phase motor is wired up and connected to voltage, there exists a 50% chance that it will turn the wrong way. So, when the installer first starts the motor, the proper procedure is to "bump" it. That is, connect it for a very short time (usually accomplished with the "Hand" or "On" position of the starter). The pump will begin to rotate. By observing the rotation of the shaft, the installer can tell if it matches the rotation arrow on the pump casing. Note that it is extremely important to fill the system first, as the pump must be full of fluid to avoid damage to the seal! If it does not, the solution is to switch any two of the three power leads. This can be done at the motor, but it is often more convenient to do it at the motor controller (starter of VFD). Very occasionally, this is not done, or somehow phase reversal later happens UPSTREAM of the motor in question. A common myth is that in reverse rotation, the pump causes backward flow, that is, IN the discharge and OUT the suction. In reality, a pump operating in reverse rotation because of wiring or phase change will pump in the normal direction. Now, it won't pump very well. Its flow will be reduced, as will its head. The NPSH characteristic will not match the cataloged value. The pump may be noisier than normal. Operation in this mode may cause several problems. Flow switches or differential pressure switches at boilers or chillers may fail to close. Hot water boilers may cut out on high limit (and chillers on freeze stat), as the flows are so small that the temperature change thru the boiler or chiller is excessive. I was once on a project where the boilers had cycled on high limit millions of times because of extended reverse rotation! In any case, the reduced flow and head will probably lead to a building or process that does not heat or cool properly. So, checking rotation on startup really is important. The seal may leak and even be damaged after a period of operation (though I have not personally witnessed this). The biggest problem might occur in a parallel pumping application where the other pump operates normally. The higher head developed by the properly-operating pump would cause the "backward" pump to operate at shutoff condition, likely eventually damaging the backward pump (see our Course HYD-150, section on parallel pumping with dissimilar pumps). Go ahead--you can read it without buying the course.
2) The other reason for reverse rotation can sometimes happen in parallel pump installations where one pump is off. Short circuiting of flow thru the off pump should be prevented by a check valve. If the check valve leaks, reverse flow from the discharge to the suction connection WILL occur in the "off" pump and this WILL cause the pump to rotate in reverse. The rotational forces and the speed of rotation depend upon the amount of the check valve leak, the head differential, and the impeller design. Again, a seal leak may eventually occur but the BIG problem is when the reverse-operating pump is called upon to start. If the reverse-rotation speed is at all significant, significant stresses will be placed upon the shaft, the coupling and the motor. Think about throwing the car into drive while backing up rapidly! Failure of the coupling and/or motor is possible. So, while I can't think of a specific myth regarding THIS type of reverse rotation, I can say that it's important to take care of it when it occurs. So, it is a good idea for a maintenance person to take a quick look at "off" pumps in parallel installations to be sure that reverse rotation is not occurring.
Any comments relating to YOUR experiences with reverse rotation are certainly welcome.
I picked up a copy of the 2011 book, Super Freakonomics by Levitt and Dubner yesterday. I had very much enjoyed their Freakonomics book some years ago. I found the chapter on Global Warming extremely interesting. While not an anti-global warming piece, it did offer what I would call a very balanced look at the whole subject. It would be folly to try to summarize many pages in a short blog, I would encourage you to read at least this chapter. If you are concerned about global warming, you may wish to purchase the book or pick it up at your local library (I don't receive a commission!). I will say that there would appear to be three or four very technically-feasible, freakishly simple, and very affordable ways to deal with global warming, even if the dire predictions of many "warmists" prove to be true (although the chapter does seem to cast a bit of doubt about the accuracy of current models). Again, this is not a pro or anti global warming piece. I found it unique in its balance and enjoyed very much reading about the innovative solutions that some very smart scientists are working on behind the scenes. It will be interesting to see how these ideas pan out.
For most homeowners hot water needs is a fleeting thought in an overwhelming series of multiple thoughts throughout our day. We think about it briefly when we wait for the shower to get warm, do some dishes, wash our hands, or even do a load of laundry. Perhaps we even ponder it when we pay our water/sewer bills. Finally, house guests can be that tipping point when we decide it may be time for a more efficient or larger hot water heater. But wait. There are many less expensive and more environmentally friendly options in today's residential plumbing world.
Water heaters have come a long way in increasing their efficiency. In conjunction with this, recirculation pumps for hot water have managed to reduce power usage to the size of a small light bulb. With the help of systems that only operate when you call for them, and timers that learn your hot water useage, recirculation systems are more efficient than ever before. Gone are the days of taxing water heaters unnecessarily and wasting water. New recirculation units have zero waste because water that is not being used immediately is sent back to the water heater via cold water and dedicated return lines-saving an average of 12,000 gallons of water a year for the family of four! Water and energy conservation continue to be crucial to the sustainability of our resources and customers are often demanding such options.
Comfort, along with conservation, is another significant advantage to the newest recirculation systems. Multiple valve options make it automatic for our system to direct the water to the right places depending on the temperature and immediacy of our needs. We tell it when to heat and how long we need it for. With these efficient units, wait time is cut by minutes when waiting for hot water. New updates in mixing valves help both residential and commercial customers mix hot and cold water more efficiently, requiring smaller amounts of hot water while still getting the same amount of water needs fulfilled.
Consumers have considered these issues, but it is our responsibility to show them the many advantages of installing, or even updating, domestic hot water recirculation systems. Whether they are interested in water savings, energy consumption, being more "green", or just want themselves and their guests to be comfortable, there are many options to choose from. Most of our systems can be installed in existing homes with very little or no disturbance. Our professional expertise helps ensure that the correct systems are selected and installed providing maximum efficiency. Please call us so we can answer any questions you may have regarding the newest options in the ever-growing segment of domestic hot water recirculation.
Most of us involved in the hydronic industry either consider ourselves knowledgeable, or are fortunate enough to know someone who is. There isn't a better or more valuable resource than a person who can get you the right information quickly and explain it clearly. Often times the internet and similar media are used to gain such knowledge, However, it's just not the same as that personal touch. Unfortunately the one-on-one teaching practice is becoming more and more rare and difficult to find. Don't worry, you just found it.
At Fluid Handling we strive to offer both the most current concepts available, as well as being a unique resource for older products such as traditional steam boiler systems. Our trainers are skilled and ready to answer the most pressing questions you may have. Sure, you may find part of an answer when you search it on your computer. But do you get the most important information, the "why", when you use that resource? More importantly, getting any follow up and unique questions answered is only possible with that human touch.
It is impossible for anyone to know all of the information available. Through classes, videos, continuous training and dedication we strive to make this bulk of knowledge manageable. Often times our classes become a collaboration of ideas between the students and the instructors. Stories are irreplaceable when it comes to solidifying a concept or idea. If you want to set your organization apart, while also managing to save time and money in the long run, contact us to set up a training session. Visit our web site at www.fluidh.com to view previous, current, and future training courses. Fluid Handling offers P.D.H. credits for the industry professionals. We are happy to customize needs as far as time, size, topics covered and specific information you may want to discuss. You can't get all that from a Webinar.
Fluid Handling Inc.
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