Estimating refrigerant charge

[lana]

Hi everybody,

I have developed a programme to estimate required refrigerant charge for a system.
The required charge includes piping volume plus evaporator and condenser volume and if there is a receiver, plus its volume.
To estimate the charge for piping volume is simple because the refrigerant condition is known. i.e. in suction line there is only superheated vapour.
To estimate the refrigerant mass in the evaporator, condenser and receiver a percentage of liquid to vapour must be assumed because both liquid and vapour are present.
For initial estimate I assumed 50%.

What do you think is more realistic?

Any comments will be appreciated.
LANA

[Argus]

It's already been done.

Look at the excel calculator on the DEFRA site. It is intended to estimate the contained charge within a system in accordance with the EU regulations on Fluorinated Gases.

http://www.defra.gov.uk/environment/...fgas/index.htm

Containment assumptions have already been refined for the component parts of a system.



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[lana]

Thanks a lot Argus.
Appreciate that.

[lana]

Dear Argus,
I tried the programme but it doesn't take into account the volume of evaporator and the condenser?
Also the suction and discharge line volumes.

Any comments?
Cheers

[Argus]

Yes, I think it does. Please read the data list in the Introduction tab.

The author has given a choice of common types of heat exchangers and piping arrangements and made assumptions on the partial phase containment in each case.

It is not a definitive tool, but one designed to give a close indication of the containment of most common system types.

The calculator was developed for the UK F Gas Stakeholders’ group who are advising on the EU Fluorinated gas regulation. This Regulation demands regular leakage inspection of all systems containing F Gases, above 3 kg, so a containment calculator is essential.

I won’t place the author’s details on the open web without his permission, but you can contact him through DEFRA or Enviros.

[lana]

Thanks Argus I will check.
Cheers

[Lc_shi]

really good link.It's helpful.

thanks

[lana]

Hi Argus,

I checked it and it doesn't take into account the volume of the condenser and the evaporator.
The calculation method is based on kg/kW.

I think this is not accurate. For a given capacity, different evaporators can be selected according to the design temperature difference. The same is applied to the air cooled condenser.

What I have done is : the internal volume of the evaporator and the condenser is considered to be the actual volume that the refrigerant must fill.

Any comments would be appreciated.
Cheers

[nh3simman]

I think this is not accurate. For a given capacity, different evaporators can be selected according to the design temperature difference. The same is applied to the air cooled condenser.

Hi lana,
I agree, it is better to calculate with actual volumes because these are known.

I assume that you are using the actual refrigerant properties. With the specified te,tc,sh,sc and Ic you can plot a vapor compression cycle stage.

The problem is, you now have to estimate what fraction of the heat exchangers are used for superheating, de-superheating and liquid sub-cooling.

I avoid the problem by allowing these to be entered. Then the problem is easy.

I know it is not 100% correct but I think that it is also safe to assume a linear phase profile in the heat exchangers.

[US Iceman]

Looks like an interesting discussion taking place...

I normally use the equipment volumes. The tricky part is determining the mass factions of the refrigerant at any one point in the equipment (and the equipment function). DX evaporators are different than flooded evaporators, and dry suction lines are very different than wet suction lines.

I don't think it is practical to assume we will find the refrigerant charge down E-3 decimal places, so why worry about it?

These are called estimates for a reason. Therefore, if we look at something that seems reasonable and it is based on some logic that appears to be valid you should be in good shape.

[lana]

Hi US Iceman and nh3simman,

I agree with you both. That is exactly my point.

First of all we call is "estimating" for a reason. The reason is we can never ever get the exact weight of refrigerant needed before charging a system.
For small systems it doesn't matter at all. We can take one or two cylinders and done with it. But for big systems it is important to know how many cylinders are needed, also it would be essential for estimating the initial cost.

I normally use the equipment volumes. The tricky part is determining the mass factions of the refrigerant at any one point in the equipment (and the equipment function). DX evaporators are different than flooded evaporators, and dry suction lines are very different than wet suction lines.

The actual liquid to gas percentage which is present at the heat exchangers are not known. I think this is the problem and only can be estimated.
What I did is :

the liquid to gas percentage in each item is an input and can be changed. When changing these percentages we can get the maximum and minimum values for refrigerant weight. For example the result is minimum 4 and maximum 6 cylinders are needed.
At least we know where we are standing.

My question was (and still is) about reasonable percentages of liquid to gas. If you think of anything then I would be glad to hear it.
Cheers

[yangchenchen]

To our experience , we charge refrigerant about 0.7kg/UST.