C/R load, Equip Selection, Commercial sales

[mad fridgie]

In these hard times, refrigeration cold/freezer rooms is not just about best engineering, it about achieving sales with a high level of reliability.
I see that there are plenty who want to know how to size and select equipment.
So here goes.
I am not going into specifics, as there is plenty of info available. (Software, but remember these programs are only as good as the information you feed it)
With general purpose rooms the heat load is normally based on a total load per 24 hours then divided by a specified run time of the refrigeration unit.
example; duty per 24hrs=240Kw
refrigeration run time= 18hrs
refrigeration duty = 13.33Kw.
In the good old days run time was normally 16Hrs, but this can be stretched to 20hrs (as lonh as other point are followed)
But how was the the daily duty calculated?
Well if you are doing this job you should know the very basics.
So lets look at air infiltration and wall transmission.
What temperature do you use?
Most use a very high figure, in NZ designers use 32C, we do get 32C (rarely) but we certainly do not get 24hours a day. Using this figure will only increase the design duty (thus oversizing the equipment required)
A more realist design temperature is 26C (still higher than the 24hr daily averages).
There are a number who do use this lower figure, and do achieve a high number sales, but they also have a significant amount of problems.
WHY, the software programs that they use are normally generated by equipment suppliers, and all input data is directly used to select their equipment.
So going back to our ambient which do you choose,
32C grossly oversized, and over priced, or 26C smaller equipment but struggles when the ambient is high. NON
Yes use the 26C for the room load, because this is a true real figure. "KW rating"
But when it comes to the equipment selection, use the duty "KW rating" but selection this duty based upon the 32C ambient, not the 26C ambient.
By now you should be asking why!
If you did select the equipment at 26C and the ambient does rise to 32C (which it does) the capacity of the equipment reduces and is unable to recover quickly enough to ensure a reasonable level of temperture control "an unhappy client" and is likley to be adding undue stress on the equipment.
If you select only at 32C your equipment will be grossly oversized, over priced, could short cycle, and you are unlikely to get the sale due to the price. (and that does become a waste of your time and money)
In life we are now trained to look at products and services that are priced mid range.
Cheapest are cutting corners
Most expensive are screwing you.
Those in the middle are offering a fair product for a fair price.
With the above design method that is actaually were you will be and you can be confident that what you have installed will reliable and efficient.
Of course you need specific desihn figure for each part of the world that you are in

[Aik]

So going back to our ambient which do you choose,
32C grossly oversized, and over priced, or 26C smaller equipment but struggles when the ambient is high.

You may select equipment at 32C ambient temperature and calculating refrigeration run time no more 22hrs. Real time of eqipment work will not be 22 hours, because ambient temperature of 32C will not be 24 hours per day.

[nike123]

What multiyear middle temperature you will use here:

Gray one is multiyear middle temperature
Light blue is multiyear midle temperature decreased for one deviation.
Blue is multiyear middle temperature decreased for two deviations.
Light red is multiyear middle temperature increased for one deviation.
Red is multiyear middle temperature increased for two deviations.
Black one is this year middle temperature.

[mad fridgie]

Hi Nike,
You need to zoom in to daily averages in summer (average daily temp for heat load calculation) maximum temp for equipment selection.

[nike123]

Hi Nike,
You need to zoom in to daily averages in summer (average daily temp for heat load calculation) maximum temp for equipment selection.

OK, here it is 27°C, but would you consider deviations or not? We get here sometimes 41°C or 42°C. It is 15K difference from daily average temperatures.

[mad fridgie]

Whats is 27C is this your maximum daily temp.

[mad fridgie]

In NZ the air temp is under 26C 99% of the time, therefore it is over 26C 1% of the time. Practically this means that the temp is above 26C for about 4 hours a day in the height summer

[nike123]

Whats is 27C is this your maximum daily temp.

Maximum daily average temperature for hottest day in October (hottest month in year)!

[mad fridgie]

Maximum daily average temperature for hottest day in October (hottest month in year)!

So it would be fair to say that your maximum temperature is some what hotter than this.
Is the 27C average over a 24hour period or over a lesser period (normal work hours for example)
So at what temperature do you design coolrooms duty in your neck of the woods and do you select the equipment at the same temperature

[nike123]

So it would be fair to say that your maximum temperature is some what hotter than this.

Yes!

Is the 27C average over a 24hour period or over a lesser period (normal work hours for example)

Over 24 hours period.

So at what temperature do you design coolrooms duty in your neck of the woods and do you select the equipment at the same temperature

Usually at 32°C and 18hr system run time. I select equipment also at 32°C temperature. My calculation tool is CalcRite software. And my quotes are rarely accepted. Therefore I think I need to reconsider my calculations.
Although, these rare customers that accept my quotes are usually happy with their cold rooms.

[mad fridgie]

Ah yes, an over sized well balanced system very rarely causes problems but is more expensive.
I would therefore size your load for 27C and 20 hours run time, but the equipment at 32C.
Have ago on one of the system you lost, and see what differnce it would make to your price. Let us all know
"Mad"

[nike123]

Ah yes, an over sized well balanced system very rarely causes problems but is more expensive.
I would therefore size your load for 27C and 20 hours run time, but the equipment at 32C.
Have ago on one of the system you lost, and see what differnce it would make to your price. Let us all know
"Mad"

Ok, I will give result when I do that.

[nike123]

I made some imaginary calculation.
This one is you suggest:



This one is as I would do it:



Difference in capacity for this example is significant and it could lead to more acceptable quote for my customer.

[mad fridgie]

Nice Nike, but still very reliable!