Low Delta T Syndrome
Low Delta T Syndrome
(OP)
What is the easiest fix for the Low Delta T syndrome in a typical primary/secondary chiller plant set up.
Reading in one of the ASHRAE Journals it was suggested to control chiller from the secondary supply water temperature rather than from the primary, if the supply water in the secondary is gettting higher than the maximum allowed temperature for 5 minutes then start another chiller. This might make sense since the DDC system will not see low delta T on the primary and start another chiller when the first chiller is not loaded 100% .
Opinions?
Reading in one of the ASHRAE Journals it was suggested to control chiller from the secondary supply water temperature rather than from the primary, if the supply water in the secondary is gettting higher than the maximum allowed temperature for 5 minutes then start another chiller. This might make sense since the DDC system will not see low delta T on the primary and start another chiller when the first chiller is not loaded 100% .
Opinions?





RE: Low Delta T Syndrome
At our facility, we use two different control methods, depending on the situation (time of year and other factors). One method we use is based upon differential pressure, which is an indication of load. The second method, which sounds a lot like what your considering we call "load factor", which is in essence a calculation of percent of full load. Under this method, we won't start another chiller until both the current "load factor" is at 110% AND the supply water temperature is above a threshold.
RE: Low Delta T Syndrome
When you say calculating percent of full load I assume that you mean chiller load. Correct?
RE: Low Delta T Syndrome
Sounds like a perfect place for pressure-independent control valves.
RE: Low Delta T Syndrome
I understand what you are saying. Also keep in mind that there is a difference between static (head) pressure and differential pressure, the former being controlled by your water makeup and the latter being a function of pump speed (flow). The ability to deliver water to the highest building will be a function of head, which is a function of both the pump head and the head (pressure) at the suction side which is influenced by the make up system.
RE: Low Delta T Syndrome
http://files.engineering.com/getfile.aspx?folder=7...
http://files.engineering.com/getfile.aspx?folder=e...
http://www.achrnews.com/articles/print/get-the-mos...
If you investigate btrueblood's suggestion, be cautious of the extra pump head needed to keep the pressure-independent valves in their control range. They can work well if designed in from the start (though I'm not a big fan of them, energy-wise). In retrofit of an existing system, you need to make sure that your existing pumps and piping can produce the head needed at the control valve location.
I think you'll find your answers in Gil's papers above.
Best to you,
Goober Dave
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RE: Low Delta T Syndrome
RE: Low Delta T Syndrome
Best to you,
Goober Dave
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