Variable frequency drive for pump

EBEBRATT-Ed

You may need to maintain flow if the fan coils and air handlers bring in outside air-freezstats and other controls can fail allowing freez-up even when flow is maintained. If the flow is reduced via a vfd this could be a problem.

Other than that reducing flow will save energy by reducing pump horsepower-you could cap all three way valve connections and control the pump speed of of water diff. pressure.

Ed

seabee570

variable speed drive for pumping application

I was wondering if a variable speed drive for a heating hot water application in a large nursing home would be feasable?The building is 275,000 sq feet,with fan coil units in patient rooms,and air handlers for offices,etc.The main heating pumps are 20 hp.The building has outdoor reset from an energy management system,but when the steam boilers were taken out,many areas were converted to fan coils,and fin tube radiation,(hot water).In these areas,the main supply and returns run in the ceilings,and they were not insulated.I know that in larger chilled water applications,vfds work,but I am not sure about hot water heating applications. I am thinking that with a vfd and reset,it might help control the excessive heat gain noticed in the office areas with uninsulated piping.Any thoughts or input?

rb_6

Energy Management Controls

The guys at the ZCP Mfg. Inc. make this ECM unit up to 75 hp.

Both fluid temperature and pressure is controlled.

VSD plus multi stage weather compensation, pump change over on duty cycles and motor failure plus remote communication and more.

Give Brian Wheeldon a call at 403.236.9560.

Tell him I said hello.

RBean

[Deleted User]

If...

you have 2 way zone valves, and you don't have a pressure activated bypass (properly sized) AND you suspect that the zone valves are bypassing due to excessive pumping capacity, then YES, I'd hop on RB's offer like white on bread.

I did a VFD on a 7.5 HP basemount pump on a district heating system for an apartment complex. The day we commissioned the VFD, the property manager came by and wanted to know if we had it on, to which I replied "YES", and he said he could tell, because it was the first time in 20 years that when it got below 50 degrees F outside that he'd EVER seen all of the windows on the apartments closed...

I say go for it. It should be mandatory on ALL pumps over 1 HP. If I were the King of all things hydronic...

ME

Tom Anderson_2

By all means install a VFD. As a general rule of thumb, expect the annual pumping costs to be cut in half.

Here are some real numbers from a real project from a real mechanical contractor I am working:

For new installations: Motors 10HP and smaller: It is less expensive to install a VFD than a motor starter.

For 25HP to 30 HP motors, total installed cost for VFD, including VFD startup service and mechanical contractor overhead and profit: $2,000. Many utilities offer nice rebates for VFD's.

Best to control through the building energy management system (DDC). Otherwise, you can connect a piping differential pressure control sensor directly to the VFD... they all have built in PID control loops to command pump speed to maintain a pressure control setpoint.

By the way, most engineers spec a VFD by-pass section. Think twice about the real need for this, keeping in mind most central pump stations have 100% backup pump capability. Is it worth the $2,300. for 25 HP motor and $2,800. 30 HP incremental cost?

VFD's have come of age.

Tom Anderson

Ike Gatlin

as a boiler manufactuer...

I always get a knot in my stomach when people start taking about VFD's on the building primary loop. I guess it’s because we consistently never see the VFD set-up properly. It is always a case of "more is better", so the primary pumps grind to a halt and the heating boilers are still trying to meet the load. The next thing you know the energy bills spike up, the boilers are constantly going off on limit and the owner generally isn’t happy with the over all operation of the system.

It is important to remember... boilers are stupid, they have no idea that the primary flow is about to abruptly stop. If you are using a VFD on the primary ALWAYS review the sequence of operation of the boiler, how are you controlling it? What tells it to start or stop? Can the boiler control react to the sudden changes in the system flow? Remember there is no marriage of the system condition to the boiler condition other than load and temperature. To the boiler load equals temperature, to the system pumps load equals pressure change.

Ike

GMcD

One thing to keep in mind though

Most pump motors use a cooling fan on the back of the motor to provide cooling, and if you run them below 65% of rated speed, motor overheats will happen. Also- look at the pump curve and system head curves and you will see that the flow and head capabilities below 60% of rated speed are minimal. A VFD mated with a chilled water or heating water pump, and the terminal units should be designed as a "whole system" based on what speeds you want to run the pump at and you may need special motors if you expect running speeds below 70% for a lot of the time. It takes some fine tuning between boiler controls, pump speed control and the system terminal controls to get it set up right. My own rule of thumb is that VFD pumping is effective for anything over 20 Hp., but minimal bang for the buck below that threshold.

Ike Gatlin

I cant believe...

none of you control savy fellows here havent jumped all over this subject today.

Ike

Brad White_9

To expand on that point

The motors in question must be inverter-duty. This is generally a function of the fan insulation and ability to withstand heat. We specify Class F as a standard and Class H if high temperature applications.

Generally if we install a drive we specify a motor replacement especially if the motor is over 5 years old, in keeping with the newest High E models. Unless we can be assured the existing motor is suitable, we change it out as a policy. Too much heartache potential if you do not.

Brad White_9

I don't think it is a matter of not jumping on it,

I think it is a matter of knowing the application and assuring that there is always adequate flow through the boiler if that is what is required. That is a good thing to watch but I think for the purposes of this thread, it is more detail which while worthwhile, does not affect the advice that VFD's have abundant merit.

Those responding I know to understand all of this so it was not brought up, is my guess.

With Viessmann Vertomats of my experience as well as Aerco's, they monitor temperatures in and out and fire accordingly. Low load, low fire. No low limit flow, they say.

Still in many systems turndown below 50% is rare so it is not like you would run them dry. Staged boilers or on-off or limited modulation? Certainly an issue, in which case you leave a low-head primary pump to keep the boiler happy. Then your higher-head distribution pump takes the prime mover role less the boiler circuit losses of course.

Horsepower being a cube function, it is the last Hz. you saved that have the biggest $avings.

My $0.02

Brad

rb_6

VFD's

are an engineered application.

One has to have an intimate knowledge of fluid hydraulics, thermal dynamics and power systems. Making the controls is the easy part…making the controls work as part of a system is a completely different story.

Thermal and power reductions vary based on numerous factors.

Sample of a typical retrofit...appx. 5 years old.

RB

Brad White_42

Exactly, RB,

Well said. Thanks.

Ike Gatlin

experiance

I will concede to you Brad that on paper it does seem simple. I will also say that what is on paper doesn’t necessarily make it to the job. We consistently see the owners of buildings exceeding the turn down of the pumps. More is better! right? As a boiler manufacturer I am saying... VFD's on the primary are great, just think the "system" through not just the operation of the primary pumps. A perfect example... we were involved in a job with a total load of 5 million, so the engineer... doing as some engineers do, speced 10 million btus worth of modulating/condensing boilers. The primary utilized a VFD, you spent all that money on modulating/condensing boilers, why not go for the full monty... anyway, now you have a system twice the size it needs to be, with a VFD. What do you think the result was? The owner, stood infront of the boilers, that he paid for, on a 40 degree day, and asked the question.... "How come all these boilers aren’t working? How come this one boiler seems to only run for a minute at a time? When one of the questions he need to ask is "why does this system pump seem to change speed faster than Clinton changing his mind?" All I am saying is... "Think it through" Any boiler is only as good as the installation.

Ike

Brad White_9

Different Engineer, Ike

I did not mean to over-simplify the concept, just that, like you, all things have to be understood. Engineering starts with books, sure, but always ends with someone flipping a switch and it has to work.

The engineer you cited did not think it through. We all make mistakes and this one he learned from, it is hoped. A sub-standard design concept ought not negate the benefits, so I am not sure what that illustrated but what not to do and as you said, to think things through.

I would add that any boiler installation is as good as it's design.

If it seems to work on paper, it should work in the field.

If it does NOT work on paper, no amount of good installation will make it so. On that we agree.

As far as VFD applications, they are a tool. You can use a tool any number of appropriate and inappropriate ways. If I use a pen knife as a screwdriver or a wrench as a hammer and disaster results, it does not mean the wrench or pen knife were not good things, just that they were mis-applied. (Not what you were saying, just making a point.)

To the subject at hand, copper fin boilers are flow sensitive so need primary pumps. The secondary side has abundant freedom and opportunity for savings. As a nation we are over-pumped I am convinced.

As Mark Eatherton said, 1 HP and above having VFD's is a very good thing.

Ike Gatlin

we are...

On the same page of the hymnal.

But I must comment on one statement you made....

"To the subject at hand, copper fin boilers are flow sensitive so need primary pumps. The secondary side has abundant freedom and opportunity for savings. As a nation we are over-pumped I am convinced."

Yes, copper-fin boilers are in fact low mass and flow is important. But that doesn’t mean the primary is free to do with as you wish. What commonly happens is a p/s arrangement, the two closely spaced tees.... the pump on the boiler begins to by pass through the tees (because the primary flow is less than the boiler flow) and with a grand total of 10 gallons of water present (in most cases) the boiler slams off on limit.

As far as the pumped world... well let’s just say I know many happy pump salesmen.

Ike

Brad White_9

We are indeed on the same page, Ike

I would still take gentle exception, more of a tweaking or semantics to terms. Primary to me means the boiler circuit, secondary to me means the radiation/emitter circuit. Terms only we each know what circuit is being referred to.

I can see the flow reversal of which you speak and how the "dumb primary pump" might start juggling hot water back to the pump suction.

One reason I find true "Commissioning" to be rare. The secondary (VFD-respondent) circuit simply has to have a low limit of flow above that point, and a lock-out. Details of control application not to be overlooked.

I have enjoyed this exchange.

Thank you and Good Knight!

OK, it is 10:50 AM but I just *had* to say that

Brad

Ike Gatlin

its called...

Interoperability specification...

I preach this to every engineer I get in front of. How does this work with that and what expectation does the designer have of each component in HIS system.

In the commercial arena, this is becoming more and more of a problem. The advent and acceptance of things like LonWork, Bacnet... and the 50 other communication protocols available out there then mix in the basics of the heating and cooling piping and operation... you get an immediate recipe for disaster. Funny thing is, I don’t blame a lot of the engineers as much as I blame the different manufactures. They all should take more interest at the design stage. You take an engineer who has to be concerned with everything in the building, understand the application, know the codes etc. Now you get some manufacturers that will walk in get him to spec the greatest thing since sliced bread.... never to be heard from again... until it comes to start-up time. Then they stand there... telling the controls guy "yes we are Bacnet... well sort of..."

Ike Gatlin
And to think someone around here wanted to call it the squire...

Brad White_9

Solid reasoning..

Interoperability Specification, I like that, Ike.

And as you pointed out, everything is Bacnet until it has to work... "we can do that" the vendors say. To which I immediately ask, "Yes you CAN. But WILL you?" Definitive yes or no please. No capabilities without action, thank you.

"Squire", huh? Sounds like a "Gentlemen's Club" north of here. Never been of course...

Dodged a marketing bullet on that one!

Brad

Tom Anderson_2

VFD Minimum Speeds

Motors driven by VFD's for HVAC variable torque applications can be run continually at very low speeeds... 5% or so... without any motor damage. As posted, the motor must be VFD rated, and that typically means class f insulation.

Motor loads in HVAC are variable torque. There are no minimum VFD motor speed limitations. None. Nada.

Constant torque loads... such as conveyer belts, etc. must have a min speed to prevent overheating. They are different animals.

When I do design review for commissioning (our line of work), I see engineers specing 30% minimum motor speeds, with no sound technical basis for that recomendation.

There is a lot of technical mythology out there... and VFD minimum motor speeds are a classic. Part of the reason for this I believe is because it is a holdover from the first VFD applications in the early 1980's... where VFD's applied to inappropriate motors did indeed cause damage at too low of a speed.

I also agree 100% that most VFD applications have the VFD setup incorrectly. Or excessive speed pressure control setpoints. I see that almost every project we do. Just as important are engineering oversites and misunderstandings.

Engineers in general do a very poor job looking at how a system works at part load.. their view is almost always looking at full load design conditions... not part load. (Having designed systems myself in past decades). And part load is where you find the performance problems. Which is why we also always commission systems at part load also.

Here in Vermont, with electric rates about $ .11 kWh, a heating pump motor for example, running 6,000 hours per year, VFD for variable flow pumping are cost effectrive for motors as small as 1 HP. And that exludes any utility rebates.

As I mentioned elsewhere, it is less expensive these days to install a VFD instead of a constant speed motor starter for motors 10 HP and smaller. My numbers are from a real project and a real mechanical contractor.

Brad White_43

Sound advice, Tom

As a design-side engineer, I can elaborate on where the 30% (or whatever but not 5%) minimum speed comes from. Part of it is with experience with motors that were not suited to the load, sure. Once that was solved and Inverter-Duty motors became universally available, the limitation went to the sensor resolution at the controlling variable.

As one common example, variable primary pumping on a chilled water system. Absent a good flow meter, delta-P was the variable, weighted against temperature. Given the square-root drop off in DP versus flow, the minimum pump speeds on paper were lower than what was commissioned in the field. Better sensors, better turn-down I think you would agree.

I had not heard of "no limitation on turndown on V. Torque motors" and will push that envelope on subsequent projects.

And your point about VFD's versus starters first cost is spot-on. Foolish NOT to use VFD's. The $ delta is less money than a sheave change.

Another application for VFD's I have used for years now is actually in Constant Volume air systems, particularly small ones, maybe 5-7.5 HP and high variable pressure drops. For example, animal labs with 2 or 3 stage filtration and a summer doubling of the cooling coil PD when it is wet. The total static pressure of clean versus dirty filters and a dry versus wet coil can easily be double. They upset airflow rates and pressure relationships. A VFD is an inexpensive way to maintain constant flow as the DP's change.

I appreciate your "commissioning agent" perspective.

Thanks

Brad White