View Full Version : Refrigeration system calculations

30-07-2015, 08:25 AM
Hello everybody,

I'm from Lithuania working in AHU's manufacture company and i'm quite new here. My company hired me for cooling block project (refrigeration system: evaporator, compressor, condenser, exv and so on) concept creation. Block is capable of cooling and heating (4way valve). I've been looking, collecting for any information through the internet for quite long time (nearly year), most of the time it redirects me to this forum. I've got a few question for You and I hope You could provide me answers.

I'm going to share you by the whole selection process that I'm doing and then provide questions (where the information is false, please correct me :)).

From the clients, we've got request where's the main information like this:

Here's the main information like air flow, temperatures-humidities in-out at sumer and winter periods and desired temperatures (for cooling and heating periods).

The whole selection process is made in such way:
1. Our coils provider has a coil selection program, where you input information (temperatures in-out, humidity and so on) and get evaporator with cooling capacity by clients demand (desired temperature).
2. By evaporator cooling capacity i'm picking up compressor.
3. Like in first step, i'm using the same program for condenser selection, where cooling capacity + compressor power input = condenser capacity.

Everything is ok for cooling process, but when it comes to heating I face the problem. When 4way valve reverses into heating process evaporator becomes condenser and condenser becomes evaporator. The problem is that, that selected equipment don't ensure client heating demand.
I think that the problem is that there's too large temperature difference (in cooling and heating season), by that reason selected equipment couldn't ensure correct demands. I think, that to solve such problem I need to put additional heater or cooler (it depends which demand conditions isn't achieved). If I'm wrong please correct me, because otherwise I don't know how to find that balance point between those two processes).

I'm also reached the conclusion (I think so), that calculations that I'm making is wrong, because I'm starting to picking equipment from cooling. It's bad because I don't evaluate cooling and heating demands. I think I should do that, because which demand is bigger, that would be the starting point and the second process - subordinate. Am I right?

In internet there's a lot of information for cooling, heating loads for buildings, but that information is inadequate for me, because from client I get such form with air flow, in-out temperatures, humidities and so on. I think I should calculate those loads by calculating enthalpies difference, am I correct? Could you provide formulas?

For example, I'm determine loads and the cooling process is for primary (cooling capacity > heating capacity). Is it possible to determine correct evaporating, condensing temperatures by some charts? If not, maybe there's main components check points (like for picking correct condenser, I've found that, condensing and condensers outlet temperatures difference should be about 3 - 5 K, if it's higher or lower - wrong)?

Sorry guys for such large post, but I walked to impasse and now I need your help. Thank you for your answers in advance.

30-07-2015, 09:13 PM
If you are trying to dimension heat pump by heating demand, without use of supplementary heat source, than you probably have to much capacity for cooling in that climate area.

Client should give you calculation for both heating and cooling requirement for that conditioned space and you should size your equipment according to client wish and money. There is no universal answer.