View Full Version : Sizeing equipment. The very begining...

14-04-2011, 11:58 AM
Hello all,
I started working for myself this year and the biggest issue I seem to have over and over again is "Sizeing". Up to now I have contacted wholesalers who do the calculations for me, but thay are all so vaugue! I can litterally get a whole KW of variation between wholesalers and as you can imagine this refelcts in prices. I fear of going for the best price incase it has been undersized to achieve such a good price, I also don't want to air on the side of caution too much because of keeping quotes competitive.
I'm not asking for all the answers but could somebody give me a good starting point to which I can learn from.
I eventually want to be able to then size all my own Air-Conditioning, Cellar Cooling and Coldrooms. Would the above knowledge also enable me to then go away and work out specifics for just a condensing unit or evaporator?

I know I'm asking alot here as it seems to be a massive grey area in the industry, as I said I just want a good honest solid starting block to then learn from.

Thanks in advance BigGreenFridge

14-04-2011, 09:05 PM
For a start, do you Cool Pack, if not, why not? ;)

15-04-2011, 05:00 AM
Sizing is more art than science. There are no clear and hard outcomes. Way to many variables. A lot of it would come down to experience. I too have found variances between wholesalers. Average out the KW ratings given and can't really go wrong. I've only had one or two sizing issues after installation from hundreads. One learns from the experience.

mad fridgie
15-04-2011, 07:11 AM
This could be classed as 2 sections, firstly sizing the load, second matching equipment (the easy bit)
Sizing the load.
This is art and science, along with commumication and understanding the application.
You need to ask the client many questions, how much, how long, when, where.
best way is by an example.
How big
What temperature
Where is the coldroom ( in a air conditioned shopping mall, we can assume a constant temp)
Where is the condensing unit going (on the roof out side, very large temp swings)
When is it used most and how long for. (early in the morning for prep, then lunch and evening meals) closed up at night.
What goes in it and when and what temperture. (warm dog poo at 30C just before we close)
When do you want cooled by (before we open in the morning)
Do you want a air cutain or plastic curtain (no, why not? bla bla)
So now your are building a picture of the clients requirements, and should be used at the very beginning of your quote. "information supplied by the client" This covers you contractually.
As you can see from above, there is more than one peak load, each should be seen as seperate.
You have to be able to meet this peak load.
High use at lunch time and evening, also likely to have high ambient on the condensor. (need to allow for this in cond unit selection)
Dog poo cooling (what is its secific heat capacity, not known, make an educated guess, is it runny like water and is fully of water, if so you could a figure similar to water, no its like a wooden log, dry and full of nuts, whats is it like, get the picture) Cooling is at night lower ambient, better capacity out of the cond unit.
Does the unit need to defrost, do want it to run all the time or do want some off cycle. (excess capacity in real terms)
How much is the client not telling the truth, or how much is he not sure about.
Your picture is getting bigger.
Now select your equipment (you your nominal KW rating)
Is humidity a concern, do I need to ensure evap temp does not drop to cold. Do I need an EPR
How long is the pipe run, how does this derate the cond unit.
What changes happen to the system in all different conditions, do I need cond fan control, I am better with EEV over a TEV.
use good practices for the rest of the gear.
Do not give your design to the client, but show the benefits from your design.
High humidity no drying of dog poo.
EEV increased COP across all working ranges
Comp capacity control.
Sell your knowledge not the equipment. be your clients partner, not just the supplier, what is good for him is good for you.
best of luck, try yourself to start, but get the wholesaler to calculate as well. Do not tell him your appraoach as you will end up with the same answer. Ask why he chose what he did

15-04-2011, 07:05 PM
I totaly agree with MF and add one important thing, when you meet your client, or on the phone, do not start with "what's the problem?" but "How can I help you?

Best of luck, and remember, its only up to you.

ah fai
18-04-2011, 05:05 AM
compressor KW = system cooling capacity?