Understanding coil selection and evap td

Not something that i work on but something that i can't seem to find on google.
Say for instance in a commercial refrigeration application i select a compressor say for example 3hp for instance has a capacity of 8.5kw (coolroom) at an evaporating temp of -5c. Now if i use this same 3hp compressor for a freezer application and now has an evaporation compressor 4kw at an evaporating temperature of -25c. Now my question is how does the suction pressure/temperature change? From reading it the temperature difference of the coil, but what exactly gives a coil a certain evaporating temp? Size of pipes, length, volume etc if you get the jist. Just something that's been teasing my brain of late Thankyou

There are two opposing forces at work, i.e. compressor suction and heat input. The compressor pulls the evaporating pressure/temperature downward while the heat being transferred from the air flowing through the coil pushes the evaporating temperature/pressure upwards. If you reduce the temperature and/or volume of air flowing through the coil, there is less heat being transferred and the compressor wins the tug of war, pulling the evaporating pressure/temperature downward.

Thankyou Gary that explanation makes it alot simpler to understand. And is this also true for the high side of the system. The compressor pushes the discharge pressure upwards and the opposing force of the air flowing through the coil with less heat content tries to pull the discharge pressure downward. If that makes sense. Cheers

Yep... you got it. :)

Thanks for the help Gary. Just another question to add regarding the sizing of the coil for humidity purposes. You say as the heat load decreases so does the evaporating temp and the compressor win and pulls the evaporating temp down. But how, say for instance a 3kw system, i'd use a smaller high td coil for a low RH and a larger low td coil for a high RH would the evaporating temp change? As you're still absorbing the same heat load, just don't clearly understand how the evaporating temp changes as you put in a larger or smaller coil for the same capacity system. Again not something likely to ever work on just all keen to expand my knowledge.

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Originally Posted by mburns

Thanks for the help Gary. Just another question to add regarding the sizing of the coil for humidity purposes. You say as the heat load decreases so does the evaporating temp and the compressor win and pulls the evaporating temp down. But how, say for instance a 3kw system, i'd use a smaller high td coil for a low RH and a larger low td coil for a high RH would the evaporating temp change? As you're still absorbing the same heat load, just don't clearly understand how the evaporating temp changes as you put in a larger or smaller coil for the same capacity system. Again not something likely to ever work on just all keen to expand my knowledge.

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Hi mburns

a smaller coil transfers less quantity of heat over a given time and needs the system to run for longer to acheive the same outcome as a larger coil, the increase in running time means you remove more moisture, resulting in a lower humidity within the space being cooled

You are not understanding the difference between required load of a system and the actual duty of a system.
Your required load includes moisture removal, the system is designed to do this. So what is needs to be said is "3Kw at the desired conditions." So if you had an existing system, you increase the evaporator size (both in surface area and fan volume) this will increase the suction pressure, also the compressor duty would increase. Because you now have more evap duty, you need to reject more heat, so your cond temp would also increasey. A fridge system will always reach some form of equilbrium. Any change in one part of the system will effect alll other parts.

Wakey Wakey:D I did this last night about this time answered a question not being asked
04:59 must be time to read & not write or drink plenty of coffee

You have 2 of the best here now Gary + Mad Fridgie

The surface temperature of the coil determines the point at which the coil will no longer remove moisture. If the coil surface temperature is X, then the coil will remove moisture until the dewpoint of the leaving air is X. If the coil surface temperature is X-1, then the leaving air dewpoint temperature will be X-1. A smaller coil has a lower surface temperature and vice versa.

It should be kept in mind that there are two factors in moisture removal:

a. The coil must be cold enough to do the job.
b. The system must run long enough to do the job.

Also keep in mind that it takes energy to remove moisture. Dropping the coil temp reduces the efficiency. That's the cost of dehumidification. Everything is balance and trade-offs. Not too hot... not too cold... just right.