The Hartman Company Newsletter

Issue: March, 2002
Topic: Evaluating All-Variable Speed Chiller Plants

Dear Hartman Company Newsletter subscriber,

Only a short while ago, chiller manufacturers displayed a great reluctance 
toward variable speed chillers and variable flow through chillers. Now, all 
that has changed. New "All-Variable Speed" chiller plants offer to cut energy 
costs in half compared to constant speed plants, and most leading chiller 
manufacturers are anxious to supply equipment for such plants. But facility 
operators need to approach the implementation of a new plant (or the upgrading 
of an existing one) to all-variable speed operation with some caution. This 
newsletter is intended to help facility managers get a start down the path to 
implementing these new technologies in their chiller plants.

Here is a list of references we have developed which you may wish to consult to 
help you decide when and how best to capture energy cost reductions by 
implementing or converting to all-variable speed cooling at your facility.

1. The decision whether to implement all-variable speed technologies in a 
chiller plant is an economic one. The rapid onset of this technology has led to 
a lot of confusion with many traditional engineering firms not able to provide 
much assistance and chiller manufacturers hard pressed to provide accurate 
simulations with their chiller plant evaluation programs. This month, Mr. Hartman 
has authored an article in Automated Buildings's online magazine thats purpose 
is to provide an approach by which the cost effectiveness of  all-variable speed 
technologies can be easily assessed. You may wish to read this article which is 
available at:

http://www.automatedbuildings.com/news/mar02/art/hrtmn/hrtmn.htm

Many local chiller representatives are not yet tuned into the changing landscape. 
We still hear chiller reps discouraging the use of variable flow through chillers 
and claiming that the use of variable speed chillers is not cost effective. This 
is the old training program. The new one is now coming from the manufacturers but 
in many cases has not yet tricked down to your local rep. If you want to get the 
real story of variable speed in chiller plants, you should consult directly with 
the manufacturers.

2. To get an idea of current manufacturers' thinking regarding the use and 
conversion to variable speed chillers, consider that Trane is one of the first to 
show substantial maintenance savings as well as energy reduction with variable 
speed chillers. A Trane bulletin on the subject is available at:

http://www.trane.com/commercial/equipment/afd.asp

3. When it comes to varying water flow through chillers, studies have shown that 
overall performance on the evaporator side is simply unaffected by the water flow 
rate. Only at very low flows is control stability a concern. With this caution in 
mind, plant operators are now free to eliminate decoupling loops from their plant 
to improve efficiency and increase system delta T as part of their conversion to 
all-variable speed. An excellent manufacturer's perspective on the issue of 
variable flow though chillers is available at:

http://www.mcquay.com/eprise/main/mcquaycom/designsolutions/
engineeringnewsletters/MQC_engineeringnewsletters#

4. Finally, some concern has been raised that the cost of an all-variable speed 
chiller plant must be greater than  a constant speed plant. This need not be true 
if the all-variable speed plant is carefully designed and not just designed as a 
constant speed plant with VFDs added. Because variable speed drives are 
intelligent devices, they can be directly connected to the plant control system, 
reducing controls costs. The chillers should be designed for highest efficiency 
at the actual loads with which they operate, not necessarily their peak load. When 
these considerations are made, it is easy to see that an all-variable speed plant 
that substantially reduces energy use when compared to a constant speed plant can 
be constructed for the same or less cost than the constant speed plant. An article 
by Mr. Hartman that shows how network connection of the chillers and variable 
speed drive can reduce controls cost is available at:

http://www.hartmanco.com/pdf/a34.pdf


Linda Zuvich, Editor
THC News
The Hartman Company


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                               The Hartman Company
                An HVAC Engineering & Technology Development Firm
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