Treated Fresh Air Duct

Dear All,

Nowadays, I am working in a Shopping Mall.The air conditioning in every showroom is provided by both AHU and Treated Fresh Air duct.The temperature of the air supplied through TFA duct is between 19-24 Degree Celsius.Generally, the amount of CFM supplied via TFA is approximately 1/3rd or 1/4th of the CFM supplied by AHU.

Now the question is, should TFA duct be bring down directly below the false ceiling or it should be left as it as above the false ceiling.Earlier, the designer and Project team were saying that there is no need of bringing TFA below the false ceiling and now they are saying that it should be drop down from the false ceiling.What will be right way ??

In mostly cases, TFA ducts are left above the false ceiling or bring down ??

As per my perspective,it should be bring down below the false ceiling.Otherwise TFA's CFM will get wasted.

If the AHU is providing the main air circulation in the mall then I would expect the TFA to be connected to the inlet of the AHU.

Dear Mr. Brian,

I am just talking about the showrooms and each showroom have their own AHU or FCU. In the common area of the mall, TFA ducts are put inside the AHU rooms.But what should should be done with showrooms ??

One more thing, if we put it near the inlet of AHU.. The AHU has a constant capacity and it will throw a fixed quantity of CFM in a minute, then what will be the use of feeding extra CFM to AHU (via TFA) ??

What is the general scenario ? Are TFA's just connected to inlet of AHU or their air distribution is done separately.

Thanks for your cooperation.

I think perhaps the TFA being fed directly into the space may be the more efficient having read up on the subject.

Please read the the effects of different schemes as put forward by Barcol....
http://www.barcol-air.co.uk/fan_coil-index.htm

It is common to feed fresh air make up into a common false ceiling space that the AHU draws air from, better to introduce direct to AHU, the main advantage of introducing treated fresh air in Malls is to partially pressurize structure which in turn reduces hot air being introduce through access doors , plus flushes fowl air from structure and maintaining oxygen levels etc.. toilet extracts with be a factor in volume flows, partial pressurization is important to overcome extracted air volumes. Full building structure flushing with fresh air is also common during non trading hours at night for example.

Thanks for your useful suggestions.I am posting the HVAC details of one showroom:

TFA CFM TFA TR Tonnage(TR)per unit Dehumidified CFM/unit Eqpmt No of Units TotalTR Total CFM
500 3.4 1.9 1400 CAHU 1 5.3 1900


As per the above mentioned detail, TFA has 500 CFM and AHU has 1400 CFM.

1.Considering the first case, TFA is left above the false ceiling (near the AHU ) .. How much CFM will the AHU deliver in a minute ?? Will leaving the TFA near AHU enhance the capacity of AHU ??

2. TFA is bring down from the false ceiling and air is distributed with its own ducting,separate from AHU air distribution.In this case, the total CFM supplied in a showroom will be 1900 (Correct me, if I am wrong).

While sharing the HVAC details with all clients, We(Mall Management) tell them that both TFA and AHU will serve a purpose of Air Conditioning in a store.So consider total TR & CFM (AHU+TFA)....

Thanks for your cooperation in advance.

Quote:

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Originally Posted by Sumit

Thanks for your useful suggestions.I am posting the HVAC details of one showroom:

TFA CFM TFA TR Tonnage(TR)per unit Dehumidified CFM/unit Eqpmt No of Units TotalTR Total CFM
500 3.4 1.9 1400 CAHU 1 5.3 1900


As per the above mentioned detail, TFA has 500 CFM and AHU has 1400 CFM.

1.Considering the first case, TFA is left above the false ceiling (near the AHU ) .. How much CFM will the AHU deliver in a minute ?? Will leaving the TFA near AHU enhance the capacity of AHU ??

2. TFA is bring down from the false ceiling and air is distributed with its own ducting,separate from AHU air distribution.In this case, the total CFM supplied in a showroom will be 1900 (Correct me, if I am wrong).

While sharing the HVAC details with all clients, We(Mall Management) tell them that both TFA and AHU will serve a purpose of Air Conditioning in a store.So consider total TR & CFM (AHU+TFA)....

Thanks for your cooperation in advance.

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Case 1 - The total air supplied to the occupied space will be 1400 cfm. This will consist of 500 cfm of TFA and 900 cfm from the room return. The remainder will pressurize the room and leak out or be mechanically exhausted. The temperature and humidity of the air entering the air handler cooling coil will be the result of mixing 900 cfm of air at the room conditions with 500 cfm of the treated fresh air. The air handler will deliver its rated 1400 cfm.

Case 2 – The total air supplied to the occupied space will be 1400 cfm. Consider that injecting the outside air into the room is no different that injecting it at any other point of the air handler return. Consider a box. If 500cfm is brought in to the box, then 500 cfm must be simultaneously exhausted, regardless of what goes on in the box.

The comfort of occupants in any space depends on more than just the temperature and humidity of the conditioned air supply. It also depends on the distribution of the air into the space, specifically the spacing and throw characteristics of the supply diffusers. Therefore, whether or not the fresh air is pre-treated, it should be introduced into the air handler return, in my opinion, as this gives the designer the best control over the supply air distribution. If the air handler is taking its return directly from the ceiling plenum, it would be a waste of ductwork to direct the ventilation (outdoor) air into the occupied space, since simply injecting it into the ceiling plenum will result in its being picked up by the air handler return. If the air handler return is ducted from grilles in the occupied space, then the ventilation air must be ducted into the air handler return or directly into the space.

Finally, understand that when any amount of outdoor air is injected into a building, the occupied space is pressurized and excess air must leak out or be mechanically exhausted. Remember the box above. If the cfm of the outdoor air is large enough to require more than three air changes per hour in the room, then some mechanical exhaust may be needed to avoid excessive door loads, or even popping out windows. A good rule of thumb is to mechanically exhaust 60% - 80% of the outdoor air.

Thanks a lot for your explanation.nobody can explain it in a better manner.Again thanks a lot Mr Dougheret for sharing your experience and knowledge.