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Abe
11-07-2001, 10:57 PM
Is there a quick fire method of calculating the heat load given the size of a room only, without the use of computer programs. I am calculating the size of condensing unit to fit on a freezer room of dimensions 1560mmX 2435mmX1960mm high.
Temperature of freezer room is -18 to -22 Celcius.
Product load, etc are all general with no extraordinary loadings as such.
If any techs have devised a quick method of roughly estimating the capacity of condensing unit required to do the job, I would like to hear of it.

Steve
11-07-2001, 11:42 PM
All major UK suppliers such as NRS, HRP or D&W will spec it out for you over the phone if you call them with the basic details.

Abe
13-07-2001, 08:36 PM
Asking HRP or RPW to do the calculating is taking the easy way out. What Im trying to achieve is the ability to work it out myself without having to work out complex mathematical workings. A guy told me once that to work out the capacity of an air conditioning unit required multiply the length and width of a room and multiply by 300. This will roughly give you the BTU's required for that particular room. I was wondering if other engineers use a similar calculation to work out fridge plant.

Steve C
23-07-2001, 03:24 AM
In Aussie different suppliers have books with quick select tables.

Andy
23-07-2001, 09:44 PM
Hi, I personally use a coldroom selection program, sold/supplied by Dean and Wood, Hubbard also suply one. I also have used a spreadsheet designed in house by one of our sales guys, the only draw back with this is that you need a book with specfic heats ect.
Regards. Andy.

Dan
24-07-2001, 01:45 AM
Originally posted by Aiyub
Asking HRP or RPW to do the calculating is taking the easy way out. What Im trying to achieve is the ability to work it out myself without having to work out complex mathematical workings. A guy told me once that to work out the capacity of an air conditioning unit required multiply the length and width of a room and multiply by 300. This will roughly give you the BTU's required for that particular room. I was wondering if other engineers use a similar calculation to work out fridge plant.

There are lots of rules of thumb. I am not sure what units your Aussie friend is using for room measurements. If you use that rule of thumb with feet as the unit of measurement you will wind up with 5 tons of air conditioning for a 10 by 20 room. Square feet per ton rules may have their place to weed out customers who cannot afford the equipment they are asking for before you spend a lot of effort doing a calculation. A common residential rule of thumb that I am familiar with is 400 hundred square feet per ton. This usually oversizes the a/c so you can walk away unaccused of undersizing the equipment.

With commercial walk-ins - especially large plants, even though I have seen some.... hmmm... 300 square feet per ton for coolers and 250 square feet per ton for freezers? Nothing I would hold on to. You could probably just take a good look at a commercial box sizing program and invent your own rules of thumb once you think you have limited the variables to your satisfaction.

Such programs are free and available from various manufacturers. And they have enough by-gum-and-gosh in them to make you want to stay away from adding by-gosh-and-gum on top of them.:)

750 Valve
16-11-2001, 11:27 AM
Aiyub, I once was told a figure of 375 watts per square metre (I can't understand why it wouldn't be cubic metre) of refrigerated space for medium temp (0 to 5C) coolroom for general storage. I don't know how correct this is, but you could look at the last ten general storage coolrooms you properly sized with a heat load program and average out a value per cubic metre. This could be useful in cost estimation for jobs but you would rather oversize than undersize. Also I wouldn't go sizing massive or special projects in this way but for small to med commercial could work well provided you sized for a good ambient temp. In Australia a good temp would be 40C even though outside ambients would only reach that high 4 to 5 days per year. Most clients didn't even know the coolroom needs a condensing unit - and probably has no other place for it than in a dirty loading dock 50 metres from the store or on top of the coolroom and is too cheap to pay for either the 50m pipe run or the exhaust fan needed to keep the thing running, so always allow an idiot factor if using general figures to work out loads.

Servicefrigo
14-12-2004, 08:24 PM
It-s very easy to make an ad-hoc calculation for a coldroom
30 m3 +2 *C use multiple 0.08 = 2.4kw at-10*C
60 m3 +2*C use multiple 0.o8 = 4.8kw at-10*C
chek that with any soft you want. :eek:

Regards

botrous
01-01-2005, 11:55 PM
Hey steve C how are you ?
You are talking about books for quik condensing units selection , i would be greatful if you can post some links about some digital books if it exsists .
Thanks in advance.

botrous
01-01-2005, 11:58 PM
In Lebanon we use this quik method to estimate heat gain for "Air Conditioning" , the method is to multiply the surface of the room in square meter by 600 BTU/H as long as the heigh of the room is 3 meters and it isn't a roof and isn't directly heated by the sun , it the last 2 cases exsists we multiply by 900BTU/H or by 700 BTU/H according to experience estimation and it seems that this method is working quiet properly for most of the cases.

Gary
02-01-2005, 11:50 AM
Once the system is up and running, how do you judge whether it is undersized/oversized and by how much?

Peter_1
02-01-2005, 05:45 PM
You may all calculate what you wish, most of the factors changes constantly.
Outdoor temperatures varies within wide ranges whereby capacities from the compressor changes. So the rated capacity of the TXV changes.
Outdoor humidity changes, so again the capacity of the compressor.
Indoor humidity changes, so capacity from the evaporator.
Load changes, so running time changes.

The door opens more often then they thought, defrost cycles last longer then expected, products never enters at the same temperature, if the goods are cheap, customers buys more due to the low price, they often use it for other purposes then initially foreseen (use the freezer as a quick cooler)....

Therefore - this is my opinion - it's necessary to calculate for the maximum expected needed capacity and learn to live with the negative factors or foresee some regulations to counteract this negative effects when the unit is too big in part load conditions.

As said in a previous post, we run a pack on some cold rooms with a COP of more than 6. Negative effect of this is now that unit runs so lees that we have in two cold-rooms problems with a too high humidity level.
Mostly in the packing room on 10°C where they wrap the meat in a plastic foil and submerge it in cooking water to crimp the foil.
So there were some options: let the pack run on a lower LP during some time controlled by a humidistat, add extra heat to the rooms (electrical or waste heat of the condenser), shutting off one side wit an SV of the ceiling evaporator (horizontal type with two cooling coils on each side of the fan), ventilation with colder outside air (which is normally dryer but not the last weeks),... We chose for adding a small exhaust fan when they are using the crimping machine and electrical re-heating after the job is finished till the humidity reaches the desired level again.
This because the initial costs to install was the lowest from all possible solutions.

You never can install a correct sized unit. It's only correct sized on some days of the year.

This is valid for cold rooms, freezers, airconditioning...
A customer will rarely complain that his unit is cooling too fast in summer.
The initial cost to install the unit will be of course higher.
So, we mostly follow rules of thumb and only calculate when bigger loads has to be frozen of chilled very fast.

We once calculated an airo with 4 VRF (cooling and heating) and some 20 indoor units. We used the, or better...our rule of thumb (50 W/m³) and the architect who controlled the offers said we were to expensive. The units were too big sized compared with others.
We hadn't taken in account the special superior glazing, the special wall insulation, the indoor loads from computers and so on.
So we made a new offer and recalulated it together with Mitsubishi. The units became much smaller.
Finally, we got the job - i knew the architect very well :-) - but it began the first cold days; the complaints about the fact that the offices were too cold. We solved it to let the units run 24 hours a day and didin't use the lower night setting anymore. It took almost 3 hours in the morning to reach 21°C in the building with outside temperatures of -5°C.

German calls this "Fingerspitzengefühl' or experience.

Gary
03-01-2005, 12:44 PM
Using rules of thumb, or for that matter precise calculations, without some means of judging the results is like having a control system without a feedback loop.

Given a means of evaluating the finished system allows us to adjust our methods to the reality of the conditions in our area.

Jacek
13-08-2010, 05:40 PM
Hi, I personally use a coldroom selection program, sold/supplied by Dean and Wood, Hubbard also suply one. I also have used a spreadsheet designed in house by one of our sales guys, the only draw back with this is that you need a book with specfic heats ect.
Regards. Andy.

Andy,

How can I get these programs.

Thanks in advance.

Jacek