mad fridgie
05-06-2010, 06:04 AM
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
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