A/C calculations

[Latte]

As a novice in the A/C world can somebody help me and explain (Hopefully in simple terms) how you work out what size A/C units to fit in rooms. Is there a simple way to sort out KWR ratings ect or is is a mass of complicated calculations or ust experience

[Mark]

hello rdocwra
volume of room in cubic feet multiplied by five gives the cooling capacity required in BTU'S.hope this helps
mark

[frank]

Or you could use the CIBSE "rule of thumb" of floor area x 125w/m2 to give you a high end kw rating. (400w/m2 if it is a machine server room)

[Apprentice]

<volume of room in cubic feet multiplied by five gives the cooling capacity required in BTU'S.hope this helps>

Doesn't design temp days or construction tightness/insulation/heat gain play a part in this? How close does this get you?

Have you ever 'screwed the pooch' using this method? ;-)

[chemi-cool]

I tottaly agree with markfiddy and frank,but I would add 10% to 20% to be on the safe side. This way you are coverd in extreme wheather - works good on your reputaion!

Chemi

[Peter_1]

I made a simple Excel calculation file in the past. It is written in Dutch but if usefull, i will translate it.
But where can i add somewhere an Excel file?

[Andy]

Hi rdocwra,
you can down load a simple program fromwww.dean-wood.co.uk
that will do the job.
Kind Regards. Andy

[iceman007]

Hello rdocwra

I've seen different companies use different calculations, the simples I've used is to measure the room in metres, multiply the two measurements together and then multiply the total by 150, to get the required cooling capacity in kW, only thing is that it may need adjustment for higher heat loads

[Peter_1]

We multiply square meters by 100 for offices (sleeping rooms x 50) and the volume (m³ )x 50 for the heating capacity and never had problems with it.

[BritCit_Juve]

As a guestimate I have tended to use 150w/m2 for comfort cooling in shops cafe or offices and 189w/m2 for equipment room.
However when the budget becomes the project I try to carry ou a full psychro analysis as this can lead to the next size down (my profit) or no egg on face as it can be taken up with the client right at the start.

One thing that I find is usually overlooked however is the humidity requirements as most air conditioning equipment today has SHRs of arround 0.65 to 0.72 so the system is dehumidyfing like crazy in an unwanted and uncontrolled manner.

[shogun7]

Depending on the tyoe of room you are talking about say for resindential of average insulation properties it's about 400 to 500 sq. ft. per ton
System air flow should be between 400 and 425 cfm per ton of cooling for dry climates, and
between 350 to 400 cfm per ton for humid climates. Dry climates are defined as those with 20
inches or less of annual rainfall, and where the evaporator coil rarely removes much moisture
from the air. These are also referred to as dry-coil climates. Humid climates, or wet-coil
climates, have more than 20 inches of annual rainfall.

[chemi-cool]

hi shogun.

I agree with your air-condition prespective but under 20 inches of rain is dry???

I live in a desert, we get here about 4 to 6 inches a year,

everywhere is muddy for three months and now all is green.

10 inches is green all year.

recheck your weather sources.

chemi

[Gary]

In humid areas, with all else working properly, CFM should be adjusted to where the supply air wetbulb temp is 20-25F below the average room air temp (not return air temp).

For example, if the average room air temp is 75F, then the supply air wetbulb temp should be 50-55F.

If the supply air temp is taken close to the coil, then wetbulb and drybulb are close to the same, so an acceptable alternative would be drybulb 21-26F below room temp.

[rbartlett]

chemi

do you design to a temperature -say 21 deg'c- or to a temperature difference -say 20 deg'f?


cheers

richard

[chemi-cool]

hi richard,

although the "thumb rule" is used very often, it is important in hot wheather like we have to check what insulation does the place have.

using the ASHREA comfort chart, 24C with 50% HUM. will be good for most people.

so that what we are going for generaly.

but as you know most of the instalations are in privet homes and many times I get statements like: "I will feel good only when it will be 15C "

then, the story is different.

but yes, that how it goes here.

chemi

[Prof Sporlan]

With respect to residential a/c load calculations, ACCA's Manual J tends to rule in the states: http://www.accaconference.com/Mercha...ategory_Code=M

They also provide a Manual N for commercial load calcs: http://www.accaconference.com/Mercha...ategory_Code=M

[Peter_1]

I think they're written for the American market (no weather data's for example for Belgium or you must look for a comparable American city), units not in SI I presume, building system of houses (set up of walls for example) is not comparable from those in Belgium, U values for windows....

But I can be wrong.

In fact, they're not that expensive to give it a try. I think that after a Manual N or J calculation we will come back to the same thumb of rules values.

We don't have the humidity problems neither that many humid countries encounter.

Systems are always calculated for maximum load (occurs 10 to 15 days a year) and is then too big for the rest of the year.
Or you install a system which suits for 90% of the year but is then too small for the hottest days.

Same for the grilles: ceiling grilles.... perfect in summer but not in the winter when they blow then warm air.
If you install floor grilles, the problem reverses.

[shogun7]

rdocwra
I believe the following is straight forward and fairly simple to follow and I hope this helps your understanding a bit more.however you must understand this is only a simplified version of a professional heat load calculation.
A short method used to determine air volume : For each sq. ft. of floor space Excluding the basement, use 1 cfm. Example: If a home has 1500 sq. ft. (139m2) based on outside dimensions, the fan capacity should be 1500 cfm (43m3/min.). A typical poorly insulated home may need 12,000 btu of cooling per hr. for each 400 sq. ft. ( 37m2)of floor space So to determine the cooling load: The 1500 ft.2 (139m2) should use 1500/400 x 12,000btu = 45,000btu/hr (4 ton). Also the number of air changes in important. There should be 6 to 10 air changes /hr. So the volume of the room is 1500 ft,2 with an 8 ft. ceiling = 12,000 cu ft. (340m3) the 1500 cfm fan will move 1500cfm x 60min/hr = 90,000cu ft./hr there for 90,000 cu. ft. hr. / 12,000 cu ft of the house = 7.5 air changes/hr.
Return air ducts are very important and they should be about 20% larger in cross sectional area then the delivery duct. So if the delivery main duct to the house has a cross sectional area of say 30 in2 ( 194cm2) the return aitr duct should be 30 x 120% = 30 x 1.2 = 36 in.2 (232 cm2).

[frank]

Hi Shogun7

Whats your theory behind the return air duct having to be 20% bigger than the supply air duct? New one on me.

[chemi-cool]

hi frank ,

to et the idae, look at water pumps, inlet is always one size bigger than outlet.

the same with air.

chemi

[Peter_1]

We select outlet duct for velcoities of 4-5 m/sec, oulet on the grilles 1,5 - 2 m/sec.

Return 3m/sec on the grilles and 6-7 m/sec in the return so bigger outlet.

[shogun7]

Frank
The fan is sized to put so much cfm against a static head, say 0.5"wg, now the fan total pressure output is a combination of both inlet and outlet pressure so if the retirn presure is too hi it deminishes the amount of avialible pressure on the discharge side of the fan thereby lowering the amount of supply cfm avialible for cooling or heating. so you want to keep the return pressure as low as economically possible.This is the long explanation ...Chmi's explanatinn is much shorter and is right on!!

[frank]

Quote "The fan is sized to put so much cfm against a static head, say 0.5"wg, now the fan total pressure output is a combination of both inlet and outlet pressure so if the retirn presure is too hi it deminishes the amount of avialible pressure on the discharge side of the fan thereby lowering the amount of supply cfm avialible for cooling or heating. so you want to keep the return pressure as low as economically possible." Unquote.

Your original statement said that the return air ductwork was normally sized 20% larger than the supply ductwork and this is what I fail to understand. Now you seem to be saying that the inlet to the fan should be larger than the outlet from the fan?

As part of our air conditioning business we design, size, manufacture and install ductwork and select fans to achieve the required air flow rates. The total fan pressure is a combination of the duct static pressure, the duct velocity pressure and the pressure losses of any duct mounted restrictions such as external louvres, grilles, attenuators, dampers etc.

As the total duct pressure must be calculated for all of the ductwork system, i.e. both the inlet ductwork and outlet ductwork from the fan, I cannot see why the inlet ductwork "is normally bigger" than the outlet duct. Admittedly, the greater the cross sectional area of the ductwork the lower the velocity for a given volume and therefore a lower resistance that may allow the selection of a fan with a smaller hp (energy saving).

Ductwork is normally sized for a specific job say office ventilation (low velocity - up to 10m/s), kitchen hood extraction (medium velocity - 10m/s to 15m/s) and maybe smoke extract systems in shopping malls (high velocity - over 15m/s).

Take the kitchen extract for instance - to ensure that any grease produced by the cooking process is arrested by the filters the velocity of the air passing through the filter must be at least 10m/s otherwise the inside of the duct becomes saturated with grease that finds it's way past the filter. That design aspect and the extraction rate for the canopy will determine the required velocity within the ductwork on the other side of the filter and on the inlet side of the fan. Using your reasoning this velocity must now be increased as it leaves the fan because the ductwork will be smaller on the outlet of the fan ? why?

Each type of job will have the ductwork sized accordingly and then the fan will be selected against a PV performance curve. Why you would size the ductwork larger on the inlet side of the fan than on the outlet is beyond me

[frank]

We select outlet duct for velcoities of 4-5 m/sec, oulet on the grilles 1,5 - 2 m/sec.

Return 3m/sec on the grilles and 6-7 m/sec in the return so bigger outlet.

Hi Peter

If the volume flow is balanced for both the input air and the return air systems then by increasing the velocity of the return air the ductwork would be smaller (Vol/Vel = Area)

[chemi-cool]

hi frank,

ducts is asesitve case, you do not just get them from a software
program.
designing good duct system is an art.

I have seen many sh...ty jobs where design egineers sust scirbled
some lines, contractor did the work, fans were working and what came out was weaker than a fart.

I have seen some beautiful jobs and I always take the name and phone of the designer.

what I am trying to say is that its not black and white. not only m/s, volume, velocity and static pressure counts although this is the base of design. from this point the human brain starts to work and as you know we are not the same. some of us are brighter then others. these people if they designing ducts, everything will be a lot simple.

chemi

[shogun7]

frank
It's quite simple really All it amounts to is it is better to be at the lowest pressure drop on the inlet side of the fan then not. you can complicate it if you like but the fact of the matter is ...the less pressure drop on the inlet the more avialible on the discharge.
So frank I would like you to consider this:
If you size the retirn duct as you propose and it's a new installation every thing looks OK... but what about when the filter gets dirty(PD) and in the supply the air is 55*F at 95% RH = to 23.56 cuft/lb Ah but return at 75*f and 50% RH = 24.51
diff = ~ 1cuft/lb, in other words as air is cooled and dehumidified it's volume decreases ...smaller duct on supply then on return. so what i'm saying is the return should always be bigger area wise then supply. If 20% is to much for you then by all means use smaller but in my estimation you are not doing the customer any favors

[Peter_1]

What's jibe Marc?

Frank, you also can use the EDM motors made by Lemmens Air trade Company. You set via dip switches the required airflow and the ventilator will adapt automatically his rotating speed, regardless of the connected duct.

To be honest, we don't calculate that much ductwork and surely not vfor small airco's (only calculated on speed).
We install on some strategic points butterfly vanes or iris regulators to make adjustment possible and a speed regulator on the fan. In this way is every adjusment afterwards possible. (It's not the most cheap solution)

What about noise if you go in offices over 10 m/sec?

We also did some leak tests on buildings for the Belgium Army. (Max airchange of 1 times/day of the total volume at a certain pressure diffrence and with some smoke generators in the building to visualise the leaks)
The first one we did the supervising Army officier said 'Even the chimney at my house don't smoke that much'.

I looked in my books of filter supplier for kitchen hoods (not our working field) and they advise a maximum velocity through the filters, otherwise fumes are going that fast through the filter which is then unable to take out the grease.

Speaking of ventilation: on the last Eureka exhibition (World inventors) the gold medal was for a toilet extractor from a Belgium inventor. It's of course patented.

It's so simple that you think: 'Hey, why I never thought on this'
In a house or apartment blocs, it's sometimes not that simple to take out the odors of a toilet because you can't install ducts anymore or it's not allowed or whatever reason.

This invention has a little fan in the water reservoir. It pulls a vacuum in this water reservoir whereby the odors from inside the stinky toilet are extracted via the water overfeed tube. The pulsation side of the extractor is connected after the siphon direct to the stinky side (What an explanation)

I also voted for him while I was there. He has a setup with a toilet and made smoke in it until you couldn't see the toilet anymore. He switched on his system and in 20 seconds, all the smoke was gone.

See for yourself on attached figure.

What about noise if you go in offices over 10 m/sec in the ducts?
We select allways for max 30 dBA.

[chemi-cool]

peter,

thats a grand idea, that toilet air system.

I think its from eating too much chocolate. (the smell)

chemi

[chemi-cool]

peter

to jibe or gibe is mocking, making fun of.

well I guess I have to shut up when I can not express myself in english.

one day I will learn it.

chemi

[Peter_1]

Originally posted by chemi-cool
peter

to jibe or gibe is mocking, making fun of.

well I guess I have to shut up when I can not express myself in english.

one day I will learn it.

chemi

I was thinking the same when I began my explanation of the toilet story. Almost erased the text after re-reading it.

All who mails me in a private message his home adress before Friday will receive a postcard from me when I'm in Italy (skiiing) next week.

[frank]

Guys

I know how to size and design ductwork - full stop. I hear all your explanations.

Have a look at this http://www.snipsmag.com/CDA/ArticleI...,64675,00.html

Peter

I did not say that we design at 10m/s in offices, that is the limit of low velocity parameters as defined in DW144 (http://62.190.54.142/bookshop/pubs/DW144.html) the defacto level for the UK. I agree with your design aspects of between 4/5m/s in the main duct and 2.5m/s in the branch ducts.

[Brian_UK]

Frank, I think you are banging your head against an English brick wall here my son.
Facts and approx percentages will never match

[Peter_1]

Originally posted by Marc O'Brien
Postcards, by email, are email harvesting machines

I cringe every time I receive one, I always then sits and waits for the spam to follow

Have fun in Italy

I don't need a home address to send e-cards but for postcards. You know, that old fashioned paper thing, sort you have to scribe yourself, put a stamp on it and then walk or drive a whole end to the post-office box to post it.
Sending spam via post should cost me a lot of stamps.

[chemi-cool]

peter,

I think those brown envelopes you get all the time from your government agencies are SPAM.

funny but when you come to think about it, you also pay for postage!!

chemi

[frank]

Brian

You've gotta laugh - (an English thing!)

No offense Guys

[frank]

How would you Guys design ductwork for a split system where the indoor unit was an above ceiling ducted unit that could achieve no more than 75pa external resistance?

[frank]

Thats the way Marc - but what about the maths?

[Brian_UK]

Originally posted by rdocwra
As a novice in the A/C world can somebody help me and explain (Hopefully in simple terms) how you work out what size A/C units to fit in rooms. Is there a simple way to sort out KWR ratings ect or is is a mass of complicated calculations or ust experience

Going back to the original request......

Richard, you might like to have a look at these from Space-Airs site:- Rules of thumb

Cheers.