AHU Load Calculation

This is in addition to Energygod's comments. Use proper form E20 and correct method as listed in ASHRAE or carrier handbooks to carry out cooling load calculation .Plot the process in P-s chart. Now days many software's are available to reduce manual calculation

For the benefit of those readers who do not have detailed psychometric background the "4.50" constant is 0.075 Lbs per CuFt for "standard" air times 60 minutes per hour to convert CFM into Pounds per hour which is then multiplied times the differential in enthalpy (heat content of air in BTU per Pound) to get BTUs per hour heat exchange load. That value, divided by 12,000 BTU/hr (value of one "ton" of cooling) yields cooling load in tons.

By the way- since we are in the "training" mode- a "ton" of cooling is based on way back when ice was used for air conditioning. A ton of water (2,000 Lbs) melting from ice at 32F to water at 32F absorbs 144 BTU per pound (the heat of fusion for water). So- 2000 x 144 = 288,000 BTU. If you divide that by 24 hours in a day, you get 12,000 BTU per hour- or one ton of cooling effect

Hi Mr. Energygod,

Thanks for the valuable information.

Can you please let me know how & what exactly does the values 139 & 31.43 refers to?

I believe this must be the enthalpy at 104F/85% & 71F/22%. Am I assuming it as it actually is?.

Thanks.

Sir,

I am naive to this after reviewing your discussion i felt very enthusiastic nad have joined in this site.pls explain me the formulae for calculation of ahu heat load. what is that "139" and "31.43" values??

And also kindly tell me the software for this calculation.

This reply is also to Mohsinrkhan86 since I somehow missed your post.

The values "139" and "31.43" are enthalpy (given as BTU per pound of air in "I-P" units) differences between the outdoor (ambient) air which was defined by the original post.

I did not calculate the values BUT they seem reasonable.

The outdoor air was given as 104F and 80% RH- VERY hot and humid, so very high thermal content. The indoor air was given as 71F and 20% RH- "normal" BUT very dry so relatively low heat content.

As I stated in one of my responses, the one "missing" value was the actual supply air temperature AND its corresponding enthalpy. In order to have a space with that low of a RH, the supply air (which is usually close to 100% RH at the supply temperature) would have had to be VERY cold (to remove the moisture required) and then likely had reheat supplied to avoid comfort issues due to the low supply temp.

Relative humidity (RH) is by definition the amount of moisture in the air compared to how much moisture the air (at that temperature) could contain at saturation (100% RH). Without getting specific, and ONLY for this discussion, if we were to assume that air at 71F could "hold" 0.1 pounds of water for every pound of air then the defined air would have "held" 0.02 pounds of water (20% of the maximum possible) in every pound of air.

IF the space was totally "vapor-tight" so that NO additional moisture could enter the space, the supply air would also have contained 0.02 pounds of water per pound of air but it would have been "saturated" (~100% RH). BUT- it is very likely that "outside" moisture will enter the conditioned space (especially because the "very dry" air acts like a sponge for any available water vapor). SO- the actual supply air would have had to have had even lower water content- maybe 0.005 pounds of water per pound of air- to allow for the "outside" moisture that would infiltrate and mix with the space air. That means that the supply air would have been "saturated" (100% RH) with 0.005 pounds of water per pound of air. That condition would have required VERY cold air that would require reheat for the comfort mentioned earlier. AFTER the reheat, the supply air would STILL only have 0.005 pounds of water per pound of air BUT the RH of the reheated supply air would likely have been 40-50%.

AS the supply air was "diluted" by the incoming water vapor, and heated by the space conditions, the resulting ROOM air would have been 71F and 20% RH since RH falls as air temperature increases- given no more added moisture.

Back to the issue of the "missing" supply air enthalpy- It would have been very low- maybe 12-15- because of the low temperature AND moisture content. Since water vapor is able to "hold" much more heat than "dry" air, moisture content is the biggest factor in determining the enthalpy value of the air.

Hope this extended discussion helps you to understand the complexities of the original post.

I haven't checked it yet, but I'm not sure the original calculation takes into account the specific heat and density of air. The 86 tons of latent cooling seems inordinately high.

1800 sq ft and 90 tons of capacity. You may have overshot your sizing X10. You really need to rework your calculations. 20 air changes per hour??? What is this room for? Most medical buildings only require 4-6 (it can really vary) but 20??

Its a pharmaceuticals plant, and 20 ACR/Hr is requirement for clean room where all of the process work continue.

The problem is quite clear to me but need not fully.......!

I've worked in some pretty big clean rooms before for catheter and angioplasty balloon production, hard drive manufacturing, the Henkel laboratory for final customer product production, etc. I've never seen such a high requirement.

The air change requirement is like a 4 bedroom home having complete air changed every 3 minutes! That's incredible.

You might want to include floor to ceiling height and get total cubic volume, not just sq meter of floor space.

Does the entire room need this many air changes? Can you vent directly over specific areas and achieve the margins specified? Are you using powered Hepa's to charge the room and getting a positive room pressure?

As per FDA requirement minimum requirement for clean room is 20 ACR/Hr, We have different filter including HEPA and need to control room pressure. Basically its a big system and we have 5 other AHU.

I looked it up, you're right. 20 is actually a minimum for that class of room. The air changes are achieved by how many feet per minute are forced through the hepa's. So you need hepa area and airflow rates across them to determine changes per hour.

The problem is we are still in the problem........ no one provide me the solution.

Your initial post was filled with errors and indicated that you did not truly understand "your problem".

My initial post (#1) and MRSWAMY's post (#2) have given you all you need to know to "solve" your problem.

NO ONE here is going to do your calculations- we have given you all the tools you need to generate a working solution to your problem.

Access the suggested manuals, or hire a competent HVAC engineer, to get to where you want to be.

I'm not the OP, but I want to ask two related questions:

First: How do I interpret / reconcile the 20 air changes per hour with the requirement for (only) 20% fresh outside air? (I'm not stating this very well, but hopefully someone will understand my misunderstanding--if you make an air change with 20% outside air (and 80% recycled air), does that count as an air change? Or must you increase the amount of air circulated so that in one hour you get the equivalent of 20 air changes at 100% outside air, which, if I calculate correctly, means 100 air changes per hour (times 20% fresh air per change, gives you 20 air changes of fresh outside air)? (I assume / hope that is not the requirement.)

Second, is that really more than is required? I mean, I'm sure there are regulations somewhere that require that, but is that really the best way to create and maintain a clean atmosphere in the room--wouldn't fewer air changes and "better" filtering do a better job of keeping the room clean, and also use less energy, which should be a valid design criteria these days? Or is it "effluvium" generated by the process and personnel in the room that you're trying to get rid of by using so many air changes?

Finally (oops, sorry, this is the third question)--presumably this is not a--well, I don't know the right name, but one of those labs where some very dangerous biological is processed that would be a problem if it escacped, right? Because, if it was, you would also have to very carefully filter and process the exhaust air from the process, and the more there is, the more has to be processed?

All reasonable questions-

First- the 20 A/C per hour is an industry standard to assure essentially equal temperatures and humidity throughout the facility- virtually no chance of stagnant air. The 20 A/C per hour is TOTAL air circulation- for the same reasons as noted earlier. The 20% "fresh" air is likely due to either the exhaust make-up, personnel fresh air requirements and/or pressurization of the space.

Second- PROBABLY, but lots of "rules" get made because "they seem to work". A well qualified HVAC designer can easily lay out an air distribution system that will provide FULL circulation with as little as 3 A/C per hour or less- BUT over 50% of the designers out there don't meet that qualification. SO- by requiring massive air circulation, the job gets done. Regarding energy usage- AGAIN, because energy USED TO be relatively cheap, the "tried and true" method of sub-cooling and reheating massive amounts of air continues to be used, even though it wastes huge amounts of energy. The end-users are making SO MUCH money that it just doesn't "hurt" that much. Systems are available- my company makes one- that can allow the high air flows AND control energy use (and the related environmental impacts- CO2 and NOx) but we are just getting started marketing our systems and potential end-users do not know about it- or other options- yet.

Third- ANY lab that handles bio-hazards or other toxic products has very defined protocols for their use, and highly effective exhaust hoods for working with those products that discharge into HEPA filters before the air is truly "discharged" into the atmosphere. The air being circulated is also likely being sent through the same type of HEPA filters before it is sent back into the space to protect the room employees from any accidental out-of-hood releases. By the way- that is also another reason for the high air change rate- any accidental releases are very quickly "diluted" below their toxic levels before they are later captured in the high grade filters.

Hope this helps.

Re: Hope this helps.

Yes, it does--thanks very much!

you are welcome-

by the way- you accidentally clicked the "off-topic" button. If you ask them, the moderators will cancel that click. (I did it once myself).

Actually, if you're referring to my last post, I clicked that off topic intentionally--I figure that thanking you, although a nice thing to do, is really off topic.

If you're referring to some other post, I don't think I marked any others on this thread OT.

If you think back at some of the recent threads- A LOT of comments that are truly OT still get posted- at least yours was about the theme of the initial post.

I would say, that was truly amusing......

20 air change is one of the requirements not all.

G4, F7, F9, and terminal HEPA filter is required for clean room with room DP, dust collector, room exhaust, etc....... their are lots of requirements.

I understand and TOTALLY agree, BUT my post was in response to a question regarding WHY certain protocols were in place and whether it was possible to satisfy those primary criteria WITHOUT using the "tried and true" approach that is normally applied.

Whether it is POSSIBLE to meet certain Operating criteria dose not negate the primary DESIGN criteria when certain protocols are mandated by the current state of the art in Good Manufacturing Practices or other Operational Criteria.

When DESIGN criteria call for certain procedures, those procedures MUST be met. However, it is still possible to operate a facility requiring those procedures at much lower energy usage levels IF some "out of the box" solutions are used that allow meeting ALL of the OPERATIONAL criteria with some non-standard, creative solutions that allow BOTH the operational requirements to be met AND reduced energy usage.

My firm HAS developed one such solution that we will be bringing to the market place very shortly which provides ALL of the operating criteria mentioned earlier AND cuts total energy usage by 70% to 75% compared to "standard" HVAC systems serving spaces like this- plus it provides 100% sterile air.

Dear BASHIR1219,

In you posting a portion of which reads as " So, Sensible Heat: 1.085X1975X(104-71)/12000 = 5.87 Ton & Latent Heat: 4.840X1975X (139 - 31.43)/12000 = 86 Ton," and 12000 at the denominator appears to be low (I am not sure)

what is the basis.?

Thanks,

rajeswari

Interesting.

Refrigeration tons is not something I'm familiar with, but the Imperial ton is 2240 lbs. US short ton is 2000 lbs.

Please enlighten.

All "normal"- not SI- calculations are based on "short" tons- 2,000 Lbs per "ton". The "imperial" ton is basically a metric ton in English units.

There is also a JRT (Japan ton of refrigeration), based on a metric ton rather than short Imperial ton (12,000 x 2240/2000 = 13,440 Btu/h).

As a historical note, I think 12,000 is a conventional value; I have heard but not verified that a "real" ton of refrigeration is something like 12,051 Btu/h. This does not matter in a world of approximate calculations.

Cool - pardon pun - so it's a conventional approximation - so the actual conversions aren't of great importance, hence the 'dogs breakfast' in naming of units - which is what triggered my curiosity in the first inst.

Dear Mr. EnergyGod,

Thank you very much for your detailed explanation and easily understandable.

rajeswari

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