Can we use VFD's with both primary & secondary loops?
Can we use VFD's with both primary & secondary loops?
(OP)
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,
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,





RE: Can we use VFD's with both primary & secondary loops?
Are you trying to solve a problem in an existing system, or are you doing a new design?
RE: Can we use VFD's with both primary & secondary loops?
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.
RE: Can we use VFD's with both primary & secondary loops?
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
RE: Can we use VFD's with both primary & secondary loops?
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,
RE: Can we use VFD's with both primary & secondary loops?
RE: Can we use VFD's with both primary & secondary loops?
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
RE: Can we use VFD's with both primary & secondary loops?
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!
RE: Can we use VFD's with both primary & secondary loops?
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.
RE: Can we use VFD's with both primary & secondary loops?
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.
RE: Can we use VFD's with both primary & secondary loops?
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
RE: Can we use VFD's with both primary & secondary loops?
RE: Can we use VFD's with both primary & secondary loops?
RE: Can we use VFD's with both primary & secondary loops?
RE: Can we use VFD's with both primary & secondary loops?
I feel we can have a delta T based control for regulating the flow across the evaporator, with the minimum flow for safety.
RE: Can we use VFD's with both primary & secondary loops?
Regards,
RE: Can we use VFD's with both primary & secondary loops?
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???
RE: Can we use VFD's with both primary & secondary loops?
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
RE: Can we use VFD's with both primary & secondary loops?
RE: Can we use VFD's with both primary & secondary loops?
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.
RE: Can we use VFD's with both primary & secondary loops?
But first, what exactly is the "problem" with low Delta T. Of course, there is efficiency loss, i.e. it is easier to cool down something warm than something not so warm.
But if 2 way valves were installed at all AHUs, seems like we could expect a "design" delta T of around 10. By the way, I don't see any magic in the number 10, but easy to calculate with. Any comments on that one?
Not trying to sharpshoot, rather trying to learn something.
PS
RE: Can we use VFD's with both primary & secondary loops?
I am certainly not a guru in all this although I have been in air conditioning for 40 years. But when the original question was asked, the first thing I thought about was Hartman's work on the subject, so I decidided to jump in with what I had read. With regards to 10 degrees as a magic number, all the air conditioning heat exchangers I have worked with had delta t's of 10 degrees. Hartman is very big on efficiency and energy management, so am I.
So the problem according to Hartman is many chilled water systems fail to attain their design delta T loads so at the end of the distribution system it may be starved at peak periods, while at the same time the chiller plant is not able to utilize its full design capacity.
RE: Can we use VFD's with both primary & secondary loops?
Trane has been promoting this concept for years. I recommend going to their website and you can download their engineering newsletter on the variable flow primary system.