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,

 

[PacificSteve]

Imok, certainly you have learned answer to the question, and the Hartman site is great.

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

 

[imok2]

PacificSteve
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.

 

[moonpe]

I have successfully designed primary variable flow chilled water system where the water flow rate through the chillers is varied and there is NO SECONDARY PUMPS. Current chiller controllers will allow you to do this and all chiller manufacturer's now allow this as long as the rate of flow change through the chiller is kept within a certain tolerance. This is accomplished through a small bypass valve in the mechanical room to minimize this problem. Not only do you now have a totally variable flow system as Mr. Hartman describes but now you also have the capital savings of eliminating the secondary pumps (albeit the primary pumps are now larger.)

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.