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hvac-saif
29-04-2014, 10:42 AM
Hi,
I'm new to the field of the heat pumps and involved in tasks related to mathematical modeling of different dynamic system. Now for my project, I came across mathematical modeling of heat pumps. I've some set of differential equations to describe the system. But there are some parameters for which I've no idea how to measure or estimate them.
For example, mass of refrigerant in the evaporator and mass of refrigerant in the condenser. Are these given in kg or some other units? Do the mass of refrigerant remains constant within the condenser and evaporator?
Can anyone tell me where can I look for these masses?

Regards

Mayball
30-04-2014, 04:15 AM
Normally the mass of refrigerant is given as the total mass in the system, and it is given in kg or oz. The greatest mass is found in the area where the liquid is found. This is in the line leaving the condenser, the receiver (if one exists) and the line preceding the expansion valve or capillary tube which is called the liquid line. The sum of the masses within the condenser and evaporator does remain almost constant when there is no receiver, but the balance of masses may shift from the evaporator to the condenser as the evaporator cools. All of the above assumes a constant speed by the compressor.

hvac-saif
30-04-2014, 10:10 AM
Dear Mayball,

Thank you very much for your reply. I have a paper about the modeling and control of heat pumps. In that paper, the author has clearly mentioned the mass of refrigerant in the evaporator (0.165kg) and condenser(0.15kg) and I'm just curious how can one determine the masses within the evaporator block and he condenser block?

mad fridgie
30-04-2014, 12:11 PM
You need to know the component internal volume, the velocity of the mass through the component,
Know the state of the mass entering and leaving the component.
The work done buy the component, this will give us the mass flow
From this you can work out mass in the component whilst it is running. Of course any thing changes then the actual mass in the component will change.
Picking some numbers from somebodies else paper means nothing unless you know the above. "how long is a piece of string"

Mayball
02-05-2014, 06:14 AM
When I hear the word "modeling" I start to look for the assumptions used in the model. One can make the assumption that there is no sub cooling in the condenser and that there is no superheating in the evaporator, and that there is no sensible heat in the gas entering the condenser or sub cooling in line between the condenser and the evaporator. With all of the assumptions, and a few others, determining the masses in the condenser block and the evaporator block becomes possible. However, for any of these assumptions to exist in the real world would be purely coincidental.

hvac-saif
06-05-2014, 06:19 AM
Thank you for your comments.
Yes, you are right with assumptions concerning mathematical model development as far as it relates to static models. But in dynamic modeling the transient behaviour of each component is considered as well. My question was rather more from theoretical point of view. Since, I'm new in this area, my understanding of condenser and evaporator was different. I thought, for a condenser with a given volume, let say, we have 50% superheated gas and 50% saturated liquid. Can I calculate mass of the refrigerant as the product of density and volume?
Also can I assume this 50% ratio, as the efficiency of the condenser?

Mayball
07-05-2014, 02:00 AM
I would say you could make a calculation the mass of refrigerant based on superheated gas, saturated vapor, and saturated liquid. In the condenser, as the refrigerant flow through the coil, first the superheat is removed from the gas, then there is a section of increasing liquid and decreasing gas, and then a section of saturated liquid, and finally a section of sub cooled liquid. The evaporator, of course is just the opposite. I doubt that the ratio of these states would relate to the efficiency of the condenser or the evaporator.

hvac-saif
07-05-2014, 09:08 AM
Thank you Mayball.
I'll probably use the efficiency of condenser(evaporator) as a measure of the degree of subcooling(superheating). Later on, I have to validate my model on a real test system. So then I can say how good is the model and the parameters like mass of refrigerant.

Regards,