HVAC Retrofit Project

[Buildtech2]

Presently, I am studying one of the retrofit project in the middle east for upgrading the HVAC and control system. The existing cooling capacity of the 5 nos. air handling units is 335 TR and the calculated capacity is only 287 TR. I am quite confident with the heat load calculations.
The existing chiller capacity is 169 TR at 115 F ambient temperature and the maximum ambient temperature in this region is not more than 106 F. Therefore, this chiller can provide cooling capacity of 284 TR at 108 F ambient temperature which is adequate for the calculated cooling capacity of the air handling units.

Now I want to know what should be the flow rate of the chilled water pump. Will it be according to the existing load or the revised load or the to suite the chiller capacity. Please advice.

Many Thanks.

 

[Drazen]

is that student question again?

your chilled water flow is determined by needed chilling capacity and referent chiller delta t.

i believe the question should be - how ahu coils performance will be affected by reduced chilled water flow.

for that:
- work with ahu representative
or
- work with coil representative
or
- find software or calculation tables/formulas for that specific coils and calculate yourself.

than if some of your coils performance is unacceptably lower than needed due to lowered flow, you have to increase design flow to reach your needs, than you need some extra chiller capacity.

from what you said chiller capacity is somewhat lower than calculated loads. how will you handle that? you should have at least some small extra provision, not minus capacity. on the other hand, you added provision for "extra" two degrees of outdoor temperature. this is tricky territory where "educated guess" of experienced designer is needed.

in overall, this shows why retrofit projects always have to be charged significantly more.

 

[Buildtech2]

Thank Dazen,

You could say that this is a student question as this is my first retrofit project which is usually complicated as compared to the fresh new project. I have attached existing chiller verification file for your review.

The existing chiller capacity is adequate as per load calculation but the existing capacities of the air handling units are higher than calculated load. The sum of cooling loads of all existing air handling unit is 335 TR.while the calculated capacity is as per revised load calculation is only 300 TR.
As advised, I will check with ahu representative if any coil performance is unacceptably lower than needed due to lower flow but replacing the chiller capacity is a big decision which would create a huge impact on the cost.

 

[Buildtech2]

Any more inputs / suggestions on this would be highly appreciated.

 

[dbill74]

Something to keep in mind, just because AHU-1 is operating at peak capacity does not mean that AHU-2 thru 5 are at capacity.
It is not uncommon for the peak load times for the different AHU's to vary. I.e. AHU-1 peak is at 1400 in late July, while AHU-2 is at 1600 in early Aug. However, system/chiller peak is at 1500.

Another reason for the variance could be the use off-the-shelf equipment; the capacities of the AHU's selected is larger than actually needed. I.e. AHU-1 calculated demand load is 39.3 tons the installed AHU has 40 or 50 ton capacity. To buy a unit that matches the calculated load exactly is not advised, for one it would be much more expensive than an off-the-shelf unit, also it provides you an easy safety factor.

 

[ChasBean1]

500 gpm total CHW flow based on your givens.