Undersized 100 ton Condensing Unit

[Cjrac]

During commissioning of a new modulating 105 ton screw chiller, the chiller kept tripping due to high head pressure. It was found that the condensing unit capacity of 105 tons was matched to the chiller without taking into consideration the added heat of compression (approx. 20 tons). There was a 20% safety factor incorporated to the design, so would it be possible to de-rate the chiller 20% (without taking away from building comfort) in order to account for lack in condensing unit capacity and eliminate nuisance trips? Also, could the chiller be de-rated by adjusting the maximum compressor stage to 20% less than the previous max? Any suggestions would be greatly appreciated. Thanks.

 

[317069]

you have 20% safety factor, do you mean your load for example is 100 tons and chiller capacity is 120 tons.
I think you will find the better answer if you contact the chiller manufacturer.

 

[MintJulep]

The right thing to do is install the correct condenser capacity. That could be a second condenser in parallel, or rip out the undersized one and install the right one.

Since that's unlikely to happen, find a good controls programmer and limit the chiller heat rejection to never exceed the condenser capacity.

 

[Drazen]

i believe that games with chiller rating cannot help you.

chiller rejects space load, so if condenser cannot cope with pressure, that means condenser capacity cannot handle space load + chiller compressor load, the fact that chiller is oversized does not come into equation.

theoretically, smaller chiller would have somewhat smaller compressor load (capacity better adjusted to actual load), but in practice your own chiller has compressor working as much as needed with its modulation/control subtleties. it is unlikely you can improve modulation preciseness of the chiller - of course, their representative had final word on it.

so i think you cannot fool energy conservation laws (as noone else can as well)...

 

[Cjrac]

Chiller has two compressors: circuit A & circuit B, circuit A= 60 tons cooling, circuit B= 45 tons cooling. Chiller compressors have 8 steps of modulation capacity, if a few stages of modulation cut off of circuit B (by chiller representative) the chiller performance would be de-rated chiller capacity to 85 tons. The refrigerant now absorbs 85 tons from the building plus 20 tons of compressor heat. The exiting 105 ton condensing unit can now reject the 105 tons without tripping on high head pressure (when previously the chiller was absorbing 105 tons from the building and 20 tons from compressor heat). Would an energy balance not be satisfied in this case? Ein=Eout? What am I missing?. Thanks.

 

[Drazen]

i am not happy if you feel annoyed with my somewhat humorous tone, it is not targeting you, it's just attempt to stray from everyday-too-serious tone on the job, while still keeping discussion alive.

of course, it is possible that i am the one who misses something: by following, let us say, simple old engineer's common sense: if your condenser cannot keep up, there could be two major reasons:
1 - space load + compressor load is higher than condenser capacity
2 - space is overcooled

as you explained that there are many steps of capacity control, the issue of compressors drawing too much power related to cooling load is likely negligible, so the mentioned two alternatives remain.

so if we neglect "modulation inefficiency", chiller simply cannot reject more heat than space heat, so unplugging some compressor steps can cause undercooling.

you can test it the other way: switch off some zones.

 

[Cjrac]

Thanks for the help guys..much appreciated

 

[EnergyProfessional]

if derating is possible the chiller manufacturer can tell you. Is it a remote air-cooled condenser? I'm surprised it is undersized since the condenser and chiller typically come from the same manufacturer. who screwed up?

Of course they just change the program to limit the compressor speed, a later upgrade may reset that and you have tripping again. Installing a correct condneser is aproppriate.

If the reduced capacity will cool the building - your load calculation will tell you. Or just try it out. It seems to trip now anyway, so you can't make it less comfortable.

 

[cry22]

sometimes such a problem is actually an electrical engineering problem.
The chiller circuit breaker may be undersized (heck may be even the wiring is undersized) - this could produce tripping when all condensers are enabled
Have your electrical engineer check the breaker feed and see if changing it to a larger breaker can eliminate the tripping (very cheap solution - under $200.00)

Just a thought - I've had such a problem with a pump tripping once, it turned out to be the breaker was undersized.

 

[NegativeZero]

this can work but its not the best thing to do, and if the manufacture agrees and you can get everyone to sign off on it I guess its ok. I would just be concerned of any changes in interior loads and OA temps that would play a part in head pressure. As long as you can maintain a good supply temp in all estimated load conditions with out exceeding 80% capacity than you should be good to go.
it would be helpful to know the conditions when you commissioned the chiller. how long did it take to trip and what were all the readings.