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Thread: AHU Calculations

21122008, 11:33 AM #1
AHU Calculations
I have a AHU system and I am trying to establish the system performance. I actually
Have not got too much experience in AHU.
i visited some website and downloaded some software .Absolutely superb. Now trouble is that i need to know a bit more in order to be able to use it properly. I bought myself a velocity meter (digital) and it reads RH %
dry bulb, velocity fpm.I tried it on Rheem system about 1415 kw.
I got at some grilles 360 fpm, 30’C supply temp and 44 % RH. In the room the
set temp is 25 and it reached 23 without a problem and I had a 100 fpm at some other grilles and supply 23’C.Question is How to get more calculations and apply it to the software so i can get a proper results.
Read somewhere to use a measuring tape which is i presume to measure the duct size and then if i know the area, the system kw,the RH,the dry bulb, wet bulb
so how can i apply this formulae. I prefer sometimes formulas in order to understand better instead of only software which will do the maths but actually it won't show me how it is done. Also i read somewhere about the formulae calculations about round duct ...Area x r (3.14) x r (3.14)
one more Q Is there any filters in the systems so i can clean them?

21122008, 06:47 PM #2
Re: AHU Calculations
my name dyar Iwant help any proplem
dyarmohammed@yahoo.com

22122008, 12:53 AM #3
Re: AHU Calculations
There certainly should be some filters fitted.
Brian  Newton Abbot, Devon, UK
Retired March 2015 and moved house May 2016, oh the fun never stops.

22122008, 01:51 AM #4
Re: AHU Calculations
Hi Sub, this might be of used to you.
http://www.engineeringtoolbox.com/ai...rtiest_8.html
NormMy wife used to say you never listen to a word I say at least I think that what she said

22122008, 08:49 AM #5
Re: AHU Calculations
Guys thanx for the replies .Once the notorious Bruce Lee said: knowing is not enough ,we must apply
Hence my questions,i had a lot of software and stuff my simply i have not been involved in AHU before and that's what confuses me a bit and i was thinking to clarify this by applying the knowledge.I got some readings from the grille 200 fpm ,23'C supply,44 % RH
Question is do i divide the system total kw into the quantity grilles( 15 : 3=5 kw) so i can expect around 5 kw per grille and how to calculate the airflow by measring at the duct size??

23122008, 03:26 AM #6
Re: AHU Calculations
if you are measuring velocity at the grilles it can be tricky
the air comes jetting out a 'vena contracta' or something like that.
you need to apply a correction factor, one place to get them is from the grille manufacturer
Some times the correction is refered to as a free area, other times an "Ak" factor
now if you are maesuring heating, neglecting the humidty levels will not cause much of an error.
Measuring the tenperature drawn into return grilles as well as the temperature coming out of the supplies can be used to work out how much heat is being supplied to the room.
What the above method will not tell you though for sure is how much heat the air handler is transfering. My reason for saying this is there could be duct leaks drawing in untreated air if the ducts were in unconditioned sapce, the return ducts could be improperly insulated and losing heat if they were running in a cool attic, or you could have deliberate fresh ventilation air drawn into the return.
Supply ducts could lose heat as well.
if you need to know what exactly the ahu is doing then you need to measure the air going into and leaving the coil.
for cooling with dehumdifiaction it gets a little more involvedTake the V out of HVAC and you are left with a HAC job

23122008, 01:06 PM #7
Re: AHU Calculations
you need to go to your user cp, and click that it is alright for you to receive PMs on this site
Take the V out of HVAC and you are left with a HAC job

25122008, 09:20 AM #8
Re: AHU Calculations
So the best results should be air on and air off the coil .Question is why measure the duct in this case ,for me there must ot be some rule of thumb calculations about a duct size
+air velocity+%RH+friction factor+type of grille which actually restrict the air flow i bit+WB+DB...
OK' guys what do you do when you clean the filters and you want to know how the system is performing ?Last edited by SUBCONTRACTOR; 25122008 at 09:20 AM. Reason: missed a bit

25122008, 04:29 PM #9
Re: AHU Calculations
well meausiring at the diffusers then at the coil will tell you if you have duct losses for one.
measuring at the coil trells you what the unit is actually doing, measuring at the grilles tells you what the room is receiving.
When there is a big descrepancy (especially if ducts run in unconditioned space) there is room for big improvements to stop wasting energy
you move more air through a clean filter than a loaded one
I think you will be most accurate maesuring the air flow into the returnsTake the V out of HVAC and you are left with a HAC job

31122008, 07:09 PM #10

09012009, 01:35 PM #11
Re: AHU Calculations
Started to get the knack of it.Today i took some measurments return air flow,duct size,temp.,RH % and i compared to the other system.One of them was running O'K and comparing to the unhealthy one i can spot the difference.We all learn every day.

10012009, 08:35 AM #12
Re: AHU Calculations
Formulas:
Airflow (CFM) = Air Velocity (fpm) * Crosssectional Area of Opening (ft sq).
You need to ensure to keep units consistent or account for them using appropriate conversion factors.
1 ton ref = 12000 btu/hr = 3.516 kW.
The specs on your unit is typically the nominal capacity as determined by the manufacturer. To determine the theoretical performance of the system given the temp and humidity conditions you mentioned, you would need to do the following:
Measure the crosssectional opening of the duct and convert your result to feet squared; multiply this value by your fpm air velocity reading; the product will give you your air flow in CFM.
You know your room supply temp and the room RH%. You need to find out the entering (outside) air temp and RH%. Now, it gets a bit confusing for a novice: you need to consult your psychrometric chart and plot the state point for the OA conditions and the SA conditions. Get the delta h value between these two state points. Then, apply the formula for total cooling capacity:
Qtotal = 4.5 X CFM X delta h
you can convert your answer to kW and then divide this result by the nominal provided by the manufacturer, multiplied by 100, you get the percentage utilization of the system capacity.
Of course, this is a fairly simplistic view of the processes taking place here, but you asked for some formulas to curb your enthusiam so there you go.

10012009, 03:26 PM #13
Re: AHU Calculations
Brilliant stuff.This is what i am after something straight forward and quick calculations.Actually i use a software to apply the measurments.The folmulaes i need just to understand what is what.Btw can you upload a psycrometric example of what you explained.When i start adding these values i am not 100% sure whether the state point is for the room temp. or outdoor temp. and of course is it room RH% i put or ambient RH%.
Thanx

10012009, 03:41 PM #14
Re: AHU Calculations
Downloaded a few ASHRAE books,each one has about 700800 900 pages but man it is full of complicated Einstein formulaes and a small engineer with no such a brain like Einstein feels like sh** reading this stuff.

11012009, 12:35 AM #15
Re: AHU Calculations
That's the fun part of the job that we had to go through at some time
Brian  Newton Abbot, Devon, UK
Retired March 2015 and moved house May 2016, oh the fun never stops.

22012009, 07:24 AM #16
Re: AHU Calculations
I think the great art of all that is to bring it down to a few useful formulaes and make it practically appliable.Theory is a theory but at the end of the day customer don't care how much you know in theory he wants the job done and the better the application of the knowledge into practice the finer the job will be.

22012009, 01:20 PM #17
Re: AHU Calculations
the formulae are simple, the great art is accurately measuring airflow
Take the V out of HVAC and you are left with a HAC job

16092009, 02:40 AM #18
Re: AHU Calculations
Here in the USA, we use a manual D for residential duct system design. We size duct system's airflow for the cooling heatload.
I have a lot of web pages on duct design & proper sizing of duct systems for adequate & efficient delivery of airflow to each room.
Formula for finding CFM Airflow
If you can measure the air velocity coming from a duct, here is a rough ballpark formula to get the CFM:
CFM = (velocity in (FPM) Feet per Minute times the square footage of the duct area, or freeairarea (Ak) of the supplyair diffuser.
Diffuser Ak .225sq.ft., X's 650fpm velocity = 146.25CFM.
FIGURING ROOM CFM  DUCT SIZES
Do a manual J load calc first for each room
Quick method: The heat gain and Btu/hr of cooling is done for each room.
At 400CFM per/ton of cooling, 12,000BTUH / 400cfm = 30BTUH for each (CFM) Cubic Foot per Minute of Airflow.
At 450CFM per/ton cooling use 12,000 / 450cfm = 26.66BTUH per each CFM, etc.
Then Select Supply duct size by CFM, velocity, & optimal Supply Air (FR) Friction Rate.
A Room requires 3000Btu/hr divided / by 30 equals 100CFM, or around a 6" dia. RD metal duct.
You need> Five duct runs for 1.5ton unit, 18,000BTUH: (Equal room loads,ha!) 18000 / 5runs= 3600BTUH / 30 = 120CFM each 6" duct velocity 611fpm.
The larger the tonnage system the more supply runs that will be needed to deliver that CFM of airflow at proper velocities with reasonable duct sizes.
A manual D incorporates the AHU's delivery capability with the engineered duct design.  Darrell

22102009, 12:04 PM #19
Re: AHU Calculations
i can mai u with fig of CalcaULATING the Airt Velocity and more of AHU PERFORMANCE CAN U WAIT FOR ONE DAY

05112009, 04:08 PM #20
Re: AHU Calculations
how to calculate ahu perfamance?

07112009, 05:51 AM #21
Re: AHU Calculations
help me to how to control the operation of re circulating pump? iwe have 6 units of water heater (edwards), 2 units of re ciculating pump.. what devices do i need to install to trigger the pumps to run and off? please help... tnx
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