AHU configuration

[axel2004]

Looking for information related to correct AHU configuration for multi-zone purpose using VFD and zone dampers. blow-thru and draw-thru???

I search books and internet and the information is not clear. some times indicated BT other DT.

I feel the only difference its related to sensible rise and moisture, but nothing related to better fan control.

regards

 

[quark]

This topic was discussed in one of the earlier threads. I didn't see any strong point for blow through condition nor I could think of one. I prefer draw through as it is easy to drain the condensate. Somebody suggested to me that there might be more carryover of moisture in case of blow through but that didn't make sense (if you maintain the coil face velocity). Let us see what others come up with.

It doesn't matter as far as fan control is concerned.

 

[marcoh]

For a multi-zone AHU blow through is required for face/bypass control for each zone. For most other AHU would suggest draw through is a better configuration as it provides better air-off discharge.

A BT design with poor fan installation potentially blow all the air through the centre for the coil, hence localised carry-over issues.

 

[ChrisConley]

Psychometrically there are some advantages to blow through. The biggest is that the motor heat is added as reheat , rather than adding to the cooling load.

The same AHU in blow-through configuration will have better cooling performance than draw-through. The use of a profile plate to even the airflow is a good way to prevent the localized carry-over... of course then you are adding pressure drop.

 

[AbbyNormal]

hot deck cold deck are blow through

zoning a home will be blow through if you have a furnace, draw through if you have an air handler

A commercial building with air handlers (heating/cooling coils) a VFD and VAV boxes will most likely be draw through

a blow through central station will cost you more

Blow through is best for highly sensible cooling loads

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[axel2004]

My point is, Why to use DT in multi-zone application when using VFD??

Better performance of controls??

 

[AbbyNormal]

explan what you mean by mulitzone

VAV boxes?

Hot deck cold deck?

A zoned house?

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

A plenum fan makes for a good BT marcoh

when you have a housed fan like a fc or a baf, you need room for the airflow to spread out like a discharge plenum. It makes the ahu bigger and more expensive.

With big sensible loads, you will be moving less air, using less cooling and less blower motor power with a blow through.



Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[axel2004]

Thank Marcoh, The point is ...we have a typical Multi-zone AHU using face/by pass (blow-thru) to control the temperature.

But we are going to replace by Draw-thru AHU (plug fan)using VFD and keeping the section of dampers (9 zones) just after fan section.

My question is which kind of AHU configuration is better, BT or DT??

regards

 

[Dunkin]

I have been thinking of the same subject (BT or DT), but as it applies to condensing units. The advantage BT has is that the motor is in the cooler upstream air, which will enhance it's life.

 

[ChasBean1]

It's like saying what's better: Stepping on the gas or not stepping on the brake...

There were some good explanations of each above. Not entirely sure of what you still need. There's no concrete answer without knowing the rest of the system. If it's a heat recovery wheel or heat pipe, you want to blow through on the supply side and draw through on the exhaust side to prevent cross contamination from dirty to clean.

I like ChrisConley's point about reheat from the motor (you can get some extra separation between dry bulb and dew point temps) but I believe that would be from a draw through config and not a blow through config...

Go with your draw-through. The only disadvantage is draining cooling coil condensate, but this can be easily resolved by having the right trap configuration...

 

[AbbyNormal]

Chas

When you have a draw through on a big latent load you get the needed reheat.

When the load is mainly sensible, draw through forces you to over cool the air to companesate for the reheat. But in many cases, you have to move more air to stay away from the saturation curve and extra dehumidification in order to get the sensiblle cooling.

On those sensible loads the blow through heats the air first, you have more heat going into the coil, means more coil capacity, and you move less air using less brake horsepower.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[ChasBean1]

Abby, good points by you...

You could probably also carry it farther by looking at weather data in that region and figure out when it would help and when it wouldn't. E.g., DT might be more beneficial in Florida but maybe not in Canada.

 

[AbbyNormal]

I think weather only factors in on the ventilation.

My opinion is when you are running cooling and dehumidification you want to be positive pressure and you do not really worry about infiltration.

What is inside the building is important. I am a Canadian in the Caribbean, take Miami's worst case humidity design scenario and this is our typical condition.

I only really need reheat when I have a dense occupancy and more so a dense and active occupancy.

Assembly halls, theatres, night clubs, churches, and my biggest challenge of them all, the Pentacostal Church.

Dense occupancy means more latent gain, more fresh air per person, and active occupancy means enormous latent gain from people.

You draw a line with the space load SHR through your room condition and it does not matter where you are located geographically.

An office load is pretty sensible and if you are going for 55 degree supply air, you may find that an extra two degrees cooling or so ie 53 off the coil to compensate for motor reheat to have 55 leaving the unit and you have already cut the saturation curve.

To technically get the 53F off of a coil, you would have to dehumidify some more. Programs like Elite will override you and tell you to base the CFM off of 57F air out of the unit to the space.

I did a telephone switch and the space load was all sensible. With the high air flow and blow through I think I was around 60 tons for the switch room. Draw through I would have been up around 69 tons and more air moving.

When I cannot extend an SHR line through my desired room condition and cut the saturation curve, I know right away it is time for reheat/dehumidifiers

Take the "V" out of HVAC and you are left with a HAC(k) job.