Can we use VFD's with both primary & secondary loops?

[arabei]

Hi all,
I was wondering about the possibility of using VFD's with the primary & secondary loops (ie. not the fomous discussion about P-only Vs. P-S systems), and if the primary and secondary loops have different delta T and accordingly the secondary loop has a gpm greater than that of the primary loop, would using VFD's solve that problem. And what if the application can't handle any slight fluctuations in the indoor temp.
Regards,

 

[KenRad]

arabei:

Are you trying to solve a problem in an existing system, or are you doing a new design?

 

[arabei]

Hi KenRad,
I'm trying to solve a problem in a new design. But what really concerns me is the whole idea of both the primary & secondary loops having VFD's.

 

[KenRad]

I can't imagine why you would want VFDs on the primary pumps; it would eliminate much of the benefit of using p-s. What parameter would determine primary pump speed?

A secondary loop with a higher GPM than the primary is a problem. You will have a hard time maintaining a constant chilled water supply temperature to your users, because the secondary supply flow will always be a mixture of primary-supply and secondary-return water in proportion to the difference in flow. So even if your chiller has good control response and maintains a constant supply, your secondary supply will fluctuate with differences in flow or load.

Please provide more information on the system:

-describe chilled water usage (I assume this is a chilled water system) -- air handlers, process, 2-way or 3-way valves, etc.
-what is the design delta T for your users?
-what range of delta T can the chillers accomodate?

Thanks,

KenRad

 

[arabei]

so, do i understand that you're aganist using VFD's in both the primary and secondary loops. I was just trying to imagine how would the system perform, i thought for a while that the VFD's on the primary loop will feel the demand of the secondary loop (which is a higher gpm), but i think it's going to be a war between the chiller and the secondary loop. what do you see?
delta T on the chillers is 16 F
delta T on the sesondary loop is 10 F
All AHU's process 2-way valves
Regards,

 

[shioktong]

VFD at primary loop is not a good solution in order just to solve the mixing of supply & return when the secondary flow will be greater than the primary loop. My suggestion is you can place check valve at the crossover decoupler. It will probably improved your system & it cost less than placing VFD primary pumps.

 

[235zzzo]

Hello Forum, Good Evening!

A]I have some convergence with Kenrad and Shioktong, I would like to say that to use VFD's in primary-loop could be quite a big non-sense.
Concept: Primary-loops are the thermal production-loops, to vary the flowrates in this loops all the time, turns the chillers/boilres to low their efficiency, that means you loose good money, even it doesn't look like, but you are saving in one side and spend it(a lot more) in another side. Now-a-days, these equipments have themselves several stages, so they can work already in some gradually way, it depends also on the quality of your control solution. So, there´s no reasons to have VFD's in primary-loops circuits.

B]Concerning secondary-loops, in the same context. In this case the situation is different, here you have to suit yourself to your several types of needs in each secondary-loops(distribuition circuit).
You can use for each "client", 2,3-gates control valves, modulating mode, in preference.
In the case of 2-gates control valves with pumps+VFD's (variable flowrates), specially if you do have larges flowrates, better to do a pay-back analyses for each circuit.
In the case of 3-gates control valves, if the flowrates are not so large, you can use constant flowrates pumps, or those market technical solutions with several speeds in function of temperature differential. It remains a fact, to install VFD's are still very expensive, asking always for an economical analyses.
I hope this can be of some help. Good luck!
zzzo

 

[KenRad]

arabei,

As I said before, it will be very difficult to maintain a constant secondary chilled water supply temperature if your secondary flow is higher than primary flow. If you increased primary flow to match secondary, this problem would be solved. Look at the literature on the chiller, and see if it can accomodate a higher flow/lower delta T (most likely it can). Then you can benefit from VFDs on the secondary pumps by controlling pump speed to maintain a constant secondary supply/return differential pressure in your system. The DP sensors should be located near your most critical or most hydraulically remote AHU or process.

Good luck!

 

VSD can be applied to primary for variable flow applications. As an option, the VSD can be applied to secondary loops. I have also seen the application of VSD to primary where secondary exists only for sucg control sequences as head pressure control during light load start-up and with heat recovery apllications and unique chiller design arrangements for proper flow matching.

Also remember to all, regardless of the secondary delta flow and primary delta flow, the load is the load and should be the same regardless of the difference in flow and delta.

Solving the problem of low delta syndrome has been well discussed and documented and one of the solutions is to place check valve in decoupled bridge. Alternatively, proper control programming can be applied with pumping flow limits at the secondary pumps so as never to exceed the on line primary pumping flow.

 

[LeaderP]

First of all, I'd reconsider the 10F delta T you've selected for the chilled water coils. Why not match your chillers? High delta T primary with low delta T secondary is more common in central hot water plant design with secondary pumps at the building, allowing smaller water mains from the central plant to the various buildings.

If you're committed to primary-secondary, despite it being and old paradigm that's time to be tossed out in favor or distributed pumping (due to variable evaporator flow capabilities of newer DDC controlled chillers), consider the use of the "BRDG-TNDR" from the BDRG-TNDR Corp. in Ft.Lauderdale, Florida (305-584-0110). This is a solution for the energy waste created by the typical "primary-secondary" bridge which at part load, introduces unused primary chilled water into the return chilled water line, lowering the return water temperature to the chillers. Low return water temperature prevents chillers from loading fully, necessitating the use of more chillers than necesary to meet the load. The bridge tender contains a flow sensor and a throttle valve in the bridge to allow only as much chilled water flow as necessary to meet the load. Bell & Gossett now also offers a similar product.

 

the use of the combination of primary and secondary or even a third level hyrdraulic circuit can produce significant energy savings in pumping power along with the ease of zoning areas with different load patterns.
The variable flow on the different levels of hydraulic circuit will also produce reduced energy rewards ie the cubic power relationship to flowrate.
To design with multi level circuits, hydraulic seperation is required which can be achieved using fluid injection theory. An example would be a 2deg C used on a chilled ceiling system, where the total flowrate through the ceiling panels could be 150% or more, greater than the primary flow through the chillers .
regards sonofoss

 

[arfan]

I need manual of service of compressor screw York

 

[imok2]

arabie, I urge you to go to this web page :

http://www.automatedbuildings.com/

and look up Thomas Hartman's page . He has just what your looking for and many more articles of information He is a PE and has his own company. I read his stuff all the time

 

[imok2]

arfan, Why don't you call York and ask them for one?

 

[GaneshJ]

The objective of the primary pump is to transfer heat to the refrigerant in the evaporator. If the heat to be transferred is less, why do we need to circulate more than what is required ? Of course the safety requirement of having a minimum flow can be ensured, by fixing the minimum speed so that the freezing of refrigerant is avoided.

I feel we can have a delta T based control for regulating the flow across the evaporator, with the minimum flow for safety.

 

[quark]

Chillers manufacturers say 100% is the minimum flow required(ofcourse there is no other way of predicting it without damage to the chiller). Though it can be done (conceptually) it is hard convincing the manufacturers and I wasted 30 days fighting for it.

Regards,

 

[remp]

The only reason you would consider putting VFD in a primary loop was to ensure exact design chilled water flow rate through the chiller during operation. If you have say a number of chillers in the primary loop you will get slightly different water flow rates through each chiller depending how many are running at any one time, due to the hydraulic nature of a multiple chiller installation. Having a VFD coupled with a flow metre on each chiller will ensure proper CHW flow no matter the situation. This is the only reason for using VFD on a primary loop that I can see.
Personally I wouldnt be using it unless the primary loop hydraulics was complex and you have loads and loads of money to spend. Beacuse the primary loop is fairly small and has a low head loss the variance in flow rates through each chiller shouldnt be that great anyway.
Anybody agree with me???

 

[PacificSteve]

I cant think of any reason that the secondary would have higher flow requirement than the primary.

I think this is a basic premise that can be dispensed with, and the problem goes away.

I have never heard of variable primary variable secondary.

Defintely do not go with VP if you have VS.
On the old VP no secondary (I am so sick of the slanted analysis of people trying to prove which one is better) sometimes this pays off, and you can regulate flow within a range -- the min GPM is set by the chiller manufacturer, and it is probably around 70% of nominal flow -- they need turbulence to get heat transfer properly, and the max is usually limited by common sense and energy loss through head loss calcs, which are usually quite a bit less than manufacturer max GPM.

Your chiller will be more energy efficient with more flow through it. This is overly simplistic, but your ability to transfer heat is related to the efficiency.

PS

 

[imok2]

Low delta T problems are very widely experienced by chilled water distribution systems in operation today. Many of the fixes that have been suggested to mitigate low delta T offer no solution at all, only more problems. However, reconfiguring such chilled water distribution systems as primary/booster "all-variable speed" systems without decoupling lines and with return chilled water temperature limits on each load will absolutely guarantee an end to low delta T problems. Furthermore, such a system can alert operators to potential problems at loads that are under performing so that these problems can be corrected before they adversely affect the comfort of the spaces served. With a carefully developed design, an economical upgrade is often achievable that will greatly improve overall cooling system performance.

 

[imok2]

The system is called an "all-variable speed series Primary/Booster system." Here are how the rules listed above have been implemented to convert the conventional Primary/Secondary system to a Primary/Booster and solve the problems typically associated with distribution systems:

1. Eliminate all possibility of direct mixing between chilled water supply and return: The decoupling line in the primary header has been removed and the primary pumps have been converted to variable speed control. With a DDC network coordinating the primary and secondary (now called booster) pumps, the pumping systems no longer need to be decoupled. Modern chillers easily accommodate the varying flows over wide ranges (depending on chiller manufacturer), so varying the flow throughout the entire system as conditions change works very well. The primary pumps operate with their respective chillers to maintain a neutral pressure in the primary distribution header as measured by a differential pressure (DP) sensor shown at the end of the primary distribution header. Operation of the booster pumps is described below.

1. Employ a direct coupled distribution system: The booster circuit pumps are directly in series with the primary pumps. In smaller distribution systems, one set of pumps can often be eliminated making the system a primary only system. In addition to eliminating the possibility of mixing supply with return chilled water, this direct coupled configuration can save capital cost when compared to decoupled Primary/Secondary schemes because Primary/Booster configurations accommodate built-in backup without the need for redundant equipment. Consider that if a booster pump fails, the primary pumping speed can be adjusted to operate at a higher pressure and provide some level of pressure differential to any of the booster circuits until the failed pump can be repaired. Thus, there is often no need for redundancy at the booster pumping stations. All this info is avialible on my previous posting web site.