Calculating ahu capacity

Firstly, you will need to know the system resistance as this determines the fan and drive selection. Secondly you do not mention the level of filtration required. Thirdly the need to attenuate sound or to provide humidity control. All these have a bearing on the physical configuration of the unit.

After you decide on these issues you select a popular brand and make contact with them, they will be only too happy to do the maths for you.

Good shopping.

Massey

Measure the water flow-rate and do the mass x td calc,the heat extracted from the air is transferred into the water,much easier.

Sorry Leon,

Not so, you need to design the air side conditions first. Ie required volume of air is the starting point. The volume of air will be distributed via some form of ducted system and this will present the fan with its external resistance and it is only at this point that you can get an idea of the fan rating required.

The thermal needs of the air stream can be determined by the temperature change required in the transit through the AHU. Given the vast amount of product selection data available in the H & V industry I would never bother these days to do a ''back to basics'' calc on the AHU design as you will end up having to buy a packaged unit anyway.

You must always start with the air flow, that's why it is called an air handling unit.

Massey.

Massey,

I took the question as being relative to an existing system,not a design for a new system.The question itself is somewhat vague however.Judging by the information presented,the system design is already done and as I indicated,a simple measurement is all that is needed on the fluid side,there being less to do regarding latent and sensible effects,air pressure etc etc.

I am also very familiar with the AHU,having spent many years in system design and particularly their application.

I concur with your statement on the "back to basics" calc,purely through boredom of it I might add.

Hi Leon,

Good to hear from you. I was looking at things the other way round hence my reply.

If the plant/system already exists then there are possibly new issues ie is more air required, does the system not deliver the conditions required of it etc?

In this day and age of efficiency gains I spend time down sizing drives and replacing motors to save energy. The VSD is much in demand!!

Take care,

Massey.

You should be able to do this with a pychrometric chart (or tables). The WB/DB entering/leaving conditions will give the enthalpies in and out of the coil. Subtract the lower from the higher for net decrease. Other scales on the chart will tell you the density of the air. Capacity Q = density x flow x delta(enthalpy).

Base on the air flow load

=400CFM=1 R/ton or 3.517KW

Air quantity - 4.237m³/s= 8,977CFM

estimate AHU Capacity =22.5 R/Ton or 80KW

Aw