Why tons is not enough? Did you know that a 100 ton AHU could be more expensive than a 200 ton AHU?

As an engineer who has been working for

Trane Philippines
for about 6 years now, I have experienced a lot of RFQ's that seems odd and or technically unsound (no offense or anything). Some people call up our office and just asks "pare kelangan ko price for 100 ton AHU at yun lang specs ko ---100 tons!" (I need pricing for 100 ton AHU, and the only specification I know is that its 100 tons!) without any details and sometimes even the voltage is even unknown.

I know that sometimes it really is the only data people have and or the person doesn't know anything about HVAC at all but i'd rather ask for the right information before providing a quotation rather than have issues later when the equipment has been purchased but technically incomplete. A perfect example of this is an Air Handling Unit for a hospital application that needs HEPA filters. During the proposal stage it was never mentioned so naturally as the process comes along, because the equipment proposal is incomplete and the unit has a low price, the unit is purchased from us without this accessory and thus eventually someone finds out that its not included and eventually the issue arises.

Because of this, since AHU's have more issues at start-up and comissioning compared to chillers, I normally tell the story of how a 100 ton AHU could be more expensive than a 200 ton AHU. This may seem odd and may tease your heads a bit but here's a in depth version of the explanation:
Going back to Psychrometry, the capacity is given by the formula:
Note: Pardon me for the IP units (This is due to Trane being an American company)

Qtotal = (4.5 x CFM x Delta h (Enthalpy))/12000
Airflow unit = CFM
Total heat unit = tons of refrigeration or simply tons
Enthalpy = BTU/lb

Now because the data provided is only "tons" and the customer keeps on pushing for a quotation and says "Base mo na lang sa standard, 400cfm/ton, 80/67 entering, 55F/90%rh leaving etc.) -- "Base the selection on standards, 400 cfm/ton, 80/67 entering air conditions, 55F/90%rh leaving, etc.) then here comes the undeniable laws of Thermodynamics and Pyschrometry.

For this example we will use 100 tons.
From the Pyschrometric chart we get the enthalpy of the "standard conditions"

h-entering @ 80Fdb/67Fwb = 31.51 BTU/lb
h-leaving @ 55Fdb/90%rh = 22.2 BTU/lb

note: for this example we ignore the following loads:
1. Supply fan heat
2. Supply duct heat
3. Return duct heat

In this example we will assume that the 100 tons given is the total load that would be the coil load:

Now since we have all the data's we then calculate for the CFM:
Qtotal = (4.5 x CFM x Delta h)/12000
(Qtotal x 12000) / (4.5 x Delta h) = CFM

Now substituting the values we get:
(100 x 12000) / ((4.5 x (31.51 - 22.2)
[note: we invert the leaving and entering temperatures since we know already that the process is cooling and that the result would be negative since it is heat rejection]
We then get the result as CFM to be ~28,643.00

Ok, so that's it, I then will size the AHU based on 28,643 cfm. Had i used the standard "400cfm/ton" then I instead would have based it on 40,000 cfm which would be greatly oversized.
Now to answer how a 100 ton AHU could be more expensive than a 200 ton AHU then here it is:

How about an AHU with all fresh air?

So, using design values for the Philippines and the same leaving air from the previous example:

95Fdb/82Fwb - Entering air

For this we will use 200 tons.
From the Pyschrometric chart we get the enthalpy of the OA conditions in the Philippines:

h-entering @ 95Fdb/82Fwb = 45.75 BTU/lb
h-leaving @ 55Fdb/90%rh = 22.2 BTU/lb

Following the same process and now since we have all the data's we then calculate for the CFM:
Qtotal = (4.5 x CFM x Delta h)/12000
(Qtotal x 12000) / (4.5 x Delta h) = CFM

Now substituting the values we get:
(200 x 12000) / ((4.5 x (45.75 - 22.2)

Now the CFM we will get is 22,646.00.

So there it is, that's the answer, the 200 ton all fresh air AHU will only need 22,646 CFM while the 100 ton "standard condition" AHU will need 28,643 CFM.

And since Air Handlers are sized based on the CFM since the coil face area is defined by the equation:

CFM = Area x Velocity (Face Velocity)

Area = how big the unit will be = how the casing is sized = all of the components will be sized on this single measure = CFM

It is very important for us equipment suppliers to know what our customer needs and that means we need at least the relevant details so that we are able to size our machines to meet requirements. Depending on your application, if its for a critical function (Clean room, process, Server room, hospital, etc.) or not, we base also our pricing on how knowledgeable the inquirer is of systems since we are less to take risks once we have all the necessary details. Honestly, if you want the best price, then show us that you have the right information and that .you are worthy of us giving it to you. After all, we are not selling peanuts; we sell engineered equipment that costs millions.

Here's the Psychrometric Chart of the explanation above.

Knowledge x Planning x Handling Details = Success

I heard from one of my colleagues that she had shared this on Facebook and some of her friends had problems understanding the formula's I used and that I should make something that explains how the formulas were derived. Sad to say that this is not your typical Mechanical Engineering lesson on Psychrometry and what I write here is the application of what we studied back in college and thus I choose not to grant the request. What's important here, in my opinion, is that you know the concept and that you can apply it as well with what you have. Never-mind on the derivation, sure you could show off how good you are at integration, algebra and whatnot, but at work, it's barely used nowadays since everything had been programmed already in your computer.

Just know that the calculation steps come from the same laws of thermodynamics, Q=mC Delta (T) and that since air is a mixture of water vapor and air (and some other gasses - Just ignore the others) then the mass will be converted to the volume flow by virtue of its density and the specific heat used will be of the air and the water vapor. Also, since the temperatures we are dealing with in air-conditioning calculations are fairly close to each other then we could ignore conservation of mass because the density doesn't change that much and that means that we could add volume flow rates and whatnot.

The world is changing and we have to move with the trend. Time is very important and if you focus on dealing with how it was derived, it might be all good at first but since we are in a competitive world and everyone wants to be world class then we should focus on optimizing how fast we could do our jobs rather than dwelling on things that should have been left @ school.

I just want to share a short story of what global competitiveness is and what it means to move with the trend of technology:

Back in 2010, I was lucky enough to be sent by Trane Philippines to the "Graduate Training Program" GTP of Trane in their headquarters at La Crosse Wi., USA. It was a 6 months program established in 1926 (if i could remember correctly) by Trane for its Engineers in the US and in other countries.

It was just like going back to school but the program is focused on arming us engineers with tools that we could use in our jobs. Just like the formula i used in calculating the CFM; I learned it there.

In fact, our professor told us to memorize only three (3) formulas and with only those formulas we could already tackle any problem in Applied Psychrometry.

Para saating mga Pinoy, Ibibigay ko rin sainyo para umangat ang ating nalalaman pareparehas...

Qtotal (tons) = (4.5 CFM x Delta (enthalpy))/12000

Qsensible (tons) = (1.085 CFM x Delta (T))/12000 --- Temperature is Degree F

Q latent (tons) = (.68 CFM x Delta (Grains))/12000 -- Yung grains is "grains of moisture per pound of dry air"

Sabi ng professor ko dun sa US na i "tatoo" namin yan sa balikat namin at wag na wag yon kakalimutan. Advise ko rin sainyo is to do the same if indeed you want to become an HVAC engineer.

Moving on to the topic. Isa lang obserbasyon ko dun nung nag aaral kami, yung mga amboys, na perfect nila exams sa Psychrometry and or other exams din. Kaming mga Asyano (dahil mga asians ang tropa ko dun) ay na perfect din namin. Pero ang pagkakaiba is yung mga amboys na nasa top ng class namin is nagagawa nila tapusin yung exam sa loob ng bente minuto lang while kaming mga Asyano inaabot kami ng mga isang oras. So, pinagusapan namin yun ng mga tropa ko at ito mga conclusion namin:

1. Dahil english ang exam, mabilis nila maintindihan at magdiskarte.. Kungbaga ang utak natin ay nag translate pa.

2. Calculator na ginagamit namin halos lahat ay scientific lamang at sa kanila ay halos lahat naka programmable + graphical.

Isang mali, sa opinion ko, sa pag schooling natin is bawal gumamit ng hi-tech na calculator at bawal din gamitin yuong programmable. Sa totoo lang nga kahit hangang ngaun hinde pa rin ako marunong gumamit ng programmable eh. Pero dahil namulat ako na tong mga kaklase ko at mga dating estudyante don sa training na mga galing sa mga top Engineering schools (Cal Poly, MIT, Cal Tech, Penn State, etc.) ay sanay gumamit eh bakit keya saatin hinde at worse pa is saatin ay bawal gamitin???

Yan sa tingin ko kelangan din natin para maging competitive tayo. Nagbago na ang mundo, impormasyon ngaun isang click nalang. Di mo na kelangan pumunta pa sa library at mag reseach ng 5 oras. Keya kng gusto natin na maging competitive tayo, gamitin din natin ang teknolohiya katulad nila.

Just to show you that the formula does what's intended for, here's a screenshot of an electronic psychrometric chart.

Here's the cooling coil calculation screenshot (Standard condition)

OA condition : (dont worry, the result is still in tolerance +/-10%, this is due to the conversion factors the software uses and whatnot)