Study of introducing pre cool fresh air system 1

[MEP.Learner]

Hi Engineers,
As title above.
For tropical country, what is the justification to introduce pre cool air system to AC system.

Besides better humidity control, what about in term of cost and energy aspect?

What i understand is that with precool air system, the air side equipment cooling coil can be with less coil thus save cost. (Asssuming there is one centralize PAHU / DOAS to serve a few indoor unit.)

With precool air system, can the chiller plant generate higher chilled water temperature while keeping the humidity in control? (As i understand this is less possible without Precool air system.)

In overall, is it recommended to introduced precool air system?
Is there any online reference where i can study more on this?

Thanks.

 

[dbill74]

It will save energy. pre-cooling the outside air, allows your main coil to be smaller. If you're following the Int'l Mechanical code, 403.3.1.1 (2015) states

The ventilation system shall be designed to supply the required rate of ventilation air continuously during the period the building is occupied, except as otherwise stated in other provisions of the code.

If you're using Dx units, you can maintain the requisite outside air and cycle the main unit's compressor.

 

[dbill74]

This pre-cooling frequently uses an energy recovery unit that transfer heat from the building's exhaust air to the outside air providing more energy savings. Some energy recovery units have a dessicant wheel which transfers moisture from the OA stream to the EA stream which reduces the amont of cooling your main coolings need to do even further.

 

[MEP.Learner]

hi dbill74,

Thank you for your reply.
Thanks for the reference of ICC. May I know where do you practice engineering?

Both items you mentioned, "It will save energy." and "allows your main coil to be smaller which i presume, maybe lower cost" i have heard that.
But it is hard to grasp, energy saving by how much and equipment can be lower cost by how much.

And is it more likely that chilled water supply temperature can be raised higher (and more energy saving comparatively) with pre cool air system being introduced?

Thanks.

 

[SAK9]

MEP learner,
It is possible to save energy if you were to use separate chilled water systems for the Pre-Cooled AHUs and the indoor cooling AHUs. In a tropical country ventilation air (outside air) air would typically need to be cooled to around 12-13[sup]o[/sup]C for dehumidification. This would require chilled water at around 7[sup]o[/sup]C for a normal 4 row coil. As for the indoor AHU units, they need to handle mostly sensible loads and a only a limited amount of latent load from the occupants (it is quite possible that the dehumidified ventilation air can handle all of internal latent load depending on the number of occupants).The sensible cooling indoor AHUs can have a much higher air leaving temperature (say 16-17 [sup]o[/sup]C) and therefore can accept chilled water at around 10-11 [sup]o[/sup]C .At once glance it looks like a great idea as you will be able to operate a substantial portion of your chilled water system at a higher temperature saving you lots of energy. But the downside is that you require two separate chilled water production loops with separate chillers and pumps. Then there is the issue of redundancy for each loop. You may be able to resolve the redundancy issue by a single chiller with some complex interconnecting piping and right sizing of the standby chiller. You will most likely end up with a complex system which many maintainers will struggle to understand correctly and may lead to operational issues. So you need to weigh factors like energy saving, additional capital cost, complexity of the plant etc before you decide to go down this route. I have not come across such systems so far so there must be a reason why they are not very popular!

 

[KesTheHammer]

I think from an energy perspective, having a pre-cooling coil for the fresh air doesn't improve the efficiency significantly if at all.

Air to Air Heat recovery (desiccant wheel, run around coil or flat plate heat exchanger) will increase the efficiency of the system and on these systems you get economy of scale when you have fewer larger units instead of several smaller ones.

In hot humid conditions, mixing the outside air on a 20% ratio gives you cooling coils with a high latent component which takes up more space and is more expensive - I thin kit works out cheaper to do one deep coil instead of hundreds.

It's a design choice to reduce the air to "wet room temperature" (say 21°C DB/21°C WB) or to off-coil conditions (12°C/12°C) or something in between. In our case, we decided to go to wet room temperature, because then you split your latent load between the precool coil and the AHU coil.





Kestell Laurie
South Africa

 

[dbill74]

And is it more likely that chilled water supply temperature can be raised higher (and more energy saving comparatively)

No, you don't want to raise the CHWS temperature. CHW coolling coils are designed to be used with a CHWS around 40°-42°F, and is required to ensure that the coil cools the air as needed.

Cooling coils are generally designed to discharge air at around 55°F to produce supply air with a desireable absolute humidity. Since chillers are designed to produce a ΔT of 10-12°F, it is advantagous to match the cooling coil ΔT. Thus if you raise the CHWS enterin the coils, the CHWR out of the coils will incrase an equal amount. The problem is then whether you can cool the air the amount you need when CHWR temperature is 55°F+.

Energy saving on the water side are gained due to reducing the CHW flow requirements. Smaller pumps (less energy used) are needed for reduced flow rates. Also smaller chillers/cooling towers use less energy.

To answer your question about where I work, southeast USA. It's generally subtraopical and the biggest benefit to "precooling" outside air in this area is humidity control.