Chiller Size 1

[montoy]

If you want to go Air-cooled Chiller and the cooling load is 91 tons. What would happen if you select a 90-ton Chiller? Or you have to go to next size, let's say 100-ton Chiller. That's if you're using just one Chiller for the 90 tons load.

 

[MintJulep]

Chiller capacity is rated at a specific set of operating conditions. Those conditions should be (but often aren't) listed in the literature.

If your operating conditions are different then your capacity will be different.

It might be less, or it might be more.

 

[walz]

Normaly the Chiller tonnage is lesser than the air handlers capacity because of diversity. Suppose you have 5 Air handlers that have a total tonnage of 91 tonnes. Then with a diversity factor of let's say 0.9 your chiller should be 81 tones (diversified load).

But make sure that this 90 tonnes is capacity at the ambinet design temperature.

Even if this 91 tonnes is diversified load, still 90 tonnes is OK for you (its ideal). One Tonn is nothing ac compared to total. but check the capacity at ambient.

 

[mechbuild]

Chiller is rated for nominal capacity, the actual capacity may be lower than the nominal capacity depending on ambient conditions.
For example, if the nominal capacity of chiller rated at 90 tons at 90 F DB (outside conditions), then the actual capacity of this chiller will be less than 90 tons if the ambient temperature is 115 F.

Walz, can you please expalin further about selecting a reasonable diversity factor in chiller selection, is there any good thumb rule or calculation. I think, block load to be calculated to select a suitable diversity factor.

 

[walz]

Diversity Factor should be calculated by calculating the block load yes...there is a school of thought which goes for a thumb rule diversity factor but it requires experience of many projects for that locality. I personally prefer to calculate the block load (which is actually diversified plant load), and then

Diversity Factor = block load / Total air handlers load

One of the technique which i apply to calculate the block load is as below.

When we size Our cooling/heating zones/Air handlers. Each of these air handlers are calcuated at their maximum, which may or may not include the profiles for internal gains like lighting, people and equipment. In some cases, there applies the profile for Ventilation also. Each Air handler's maximum load can vary depending upon these profiles as well as their outside exposure walls and glazing. Say we have 5 AHU's,,,,their max load will occur at their own different times.

Now to calculate the block load, Consider all the zones to be served with one AHU/System. ALl the profiles will apply in the same way Collectively. When this way will be calculated, will give us the block load.

 

[Ziggypump]

Question, how did you come about the 91 ton chiller? If you ran a simulation through sizing software, hopefully the diversity has already been taken into account.

Now that you know what the cooling capacity is required, size the chiller. If the conditions work a certain way (glycol, outside air temp, flow rate. . .) a 100 ton chiller may be needed, but but only 90 tons of cooling capacity will result.

I've run into unit ventilators that are sized for 5 tons, but with a lower air flow rate, and greater mixed air temperature needed, the actual unit only supplies 4 tons (which was the goal).

Running your calculations is only part of the answer. Now you need to select a unit that will operate and meet those conditions. Just because dividing the cooling capacity by 12,000 gives you tons, doesn't mean that the equipment will work that way.

 

[marcoh]

If you need 91 tons of cooling and you install a 90 ton chiller you will run out of cooling capacity on a design day.

As per the comments above and how confident youare in you calculation I doubt it would be an issue. What safety factor have you allowed for in your cals? If you have allowoed say 10% you may only need 82 tons of cooling anyway.

But if you want to play it safe go for the 100t chiller!