Heat Loss/ Gain Calculation 10

[AUTOKAD]

It might be a funny question but here goes..I'm just curious, how do they do the heat loss calc back when theres no computer or software. How do they do it manually. I ask someone, and says 'you can only get the value on the computer, it calculates it for you'..and i say, is there a way to do it without the software?.answered no. hmmm. makes me wander..

 

[IRstuff]

What's to wonder? Why do you think one of the basic equations is called Newton's Law?

You solve them the same way a computer solves them. A computer is no smarter than a human, only faster.

TTFN

FAQ731-376

 

[cdxx139]

Q = U A dT

still works for heating, but the U value changes with cooling (because of the sun - north face vs south; time delay of sune heat etc), so you have to look it up in charts. And then you did a couple iterations to see what month and time of day was the worst case. More experience equalled less iterations.

Our computers now do an iteration for all 8700 hours of the year, and tell you the worst case.

I believe ASHRAE called this the CLTD method and was the last hand calculation method accepted. Now it is all heat transfer and time related.

knowledge is power

 

[DRWeig]

Get yourself a copy of ASHRAE Fundamentals, it's all in there.

I still do small jobs by hand...

Good on ya,

Goober Dave

 

[mauricestoker]

I used to use the TETD/TA and still keep spreadsheets for locations where I used it.

 

[cdxx139]

taken from ASHRAE Fundamentals 2009

"For a manual cooling load calculation method, refer to the CLTD/CLF method in Chapter 28 of the 1997 ASHRAE Handbook—Fundamentals"

knowledge is power

 

[JVKS]

I started out doing hand calculations. the people i trained under were not computer savvy and the only person who had a computer was the secretary.

similar to the computer programs we manually filled out sheets(room size, height, orientation, u values,etc.) and manualy crunched the numbers on a calculator. the hardest part was the the sun load. even then we had a standard sheet for the local areas and orientation.

alot of the information needed at the firm i worked for was already on paper(mimeograph!) and i still have some of it in my old files.

it was alot of work but was pretty automatic after the first couple of dozen rooms.

I learned from the manual calculations where the values came from.

 

[AUTOKAD]

Thank you all for your response. I know it's easier to do it on the computer and it's automatic..BUT sometimes it's nice to get it and learn it how did that 'value' comes out. Rather than just punching it in the keyboard. Lots to learn! Thanks again

 

[msquared48]

Autokad:

Don't blankedly trust the computer results. You need to know how to do this manually to get a feel for the program and know when and how to tweek the input to get the correct results. To do otherwise is blind engineering.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[zdas04]

To reinforce Mike's point, you need to realize that there has not been a fully tested program on the market for at least 20 years. I did an assessment of the things that could go wrong with an application I developed in 1984 and found that it was going to take a couple of hundred thousand man hours to test every combination and permutation of inputs. Wasn't going to happen (we had 10 man weeks to do the acceptance testing). That was a SIMPLE program. Something doing any HVAC calculation has many orders of magnitude more complexity.

Since no program will ever be fully tested, you can be certain that there are errors. Some will result in the program stopping (these are the "good" kind). Some will run to completion and simply give you wrong answers (these are the bad kind). If you don't have an order-of magnitude feel for your calculations then you will never know when the program is lying to you. The only way to get that feel is to be able to do the calcs on a calculator (or spreadsheet, or slide rule). I'm not advocating redoing every calculation on an Engineering Pad, but when you are expecting the answer to be "100" and it comes back "7000", you need to be able to satisfy yourself whether your expectations were wrong or the program hit one of those non-documented bugs.

The guy that said there was no way to do the calculations without the software was a VERY dangerous idiot. He needs to understand that every program was written by a person referencing calculations that were developed by (usually) another person.

David

 

[patprimmer]

David is absolutely spot on.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[stanier]

Many facilites are just copies of a similar one. Sad but true. Companies specialise in an industry and get to knwo what size chiller, air cnditioners, fans, cooling towers are used. The units are included in the estimate and sometimes no calculations are done. They fix things when they dont work when the client complains. Or sometimes they dont fix it and use their weasel words to get out of it. hen the next job is changed.

I do not advocate this approach but the building services industry is considered to be at the bottom of the pile when it comes to engineering fundamentals. Lifts and air conditioning plant is always undersized as the contractors try and save money. Sustainability is all worth less mouthings. Lip service is paid to this when the embedded energy of systems is not taken into account. Mechanical services equipment last 10 years if you are not on the coast or just get lucky. No provision is made for expansion.

 

[DRWeig]

zdas04: Well spoken, all should take heed.

AUTOKAD: It's rather exceptional that you desire to know the fundamentals upon which your loads program is based. Don't lose that desire! It's what has kept engineering interesting for me for 30+ years...

Good on ya,

Goober Dave

 

[Drazen]

Once you assume that program cannot be fully trusted, you are already on right path.

I belong to the "older gard" who did almost everything by hand at least on few small projects, and now I see young engineers who seem to be completely relying on pressing software buttons, which goes really close to driving car by looking at the map without taking a look through windshield.

The main problem is that there is less and less effort to provide basic eduction on that through regular courses that are inclined to provide very generic and oversimplified explanations.

That is why Autokad's desire to learn about process is really smart.

ASHRAE Fundamentals book gives exact overview of the process, and I would recommend that you do it manually for several typical rooms, but to avoid too many repetitions as once you get a grasp of process, you can spend your time better by reviewing software results and comparing them.

Personally, I have more trouble currently with infiltration issues for residential projects than with anything else.

After you finish several projects for your frequent location, you will have many data about losses/gains per area and orientation both for outer walls/roofs and glasses with some typical SHGF, interior loads as well, but infiltration is real burden and you actually need to assess it.

 

[willard3]

I was early to the software heat gain/loss software and I was doing it by hand at the time. I found some huge anomalies in the software and notified the vendor. I was very glad I didn't trust the software.

Since then, I have always checked computer output by hand calculation. It takes more time but weeds out many unintended buggers that you may gloss over in front of a computer. It's the difference between an Engineer and a computer jock.

ASHRAE fundamentals is the source for heat gain/loss.

 

[Badbird2000]

When i started back in 1995, we used Trane Tracer, but my company also sent me to the local Trane A/C classes for 9 weeks. They actually taught us the hand method, and it made learning the computer version actually easier for me. Takes some experiance to look at a load and tell if you did it correct or not. Never take a computer-run load for face value.. too many chances for operator error.

Nver stop learning!