5 replies to this topic

[newtech]

Hi


This is a industrial refrigeration system that uses flooded evaporator and accumulator, in the tube is of the evaporator is glycol and shell side R22 freon, freon temperature at about 45 degrees celcius and 17.64kg/cm2A from the condenser then routed through the filter dryer then pass the level control valve then to the evaporator.

Tube side is glycol at 12.8 degree celcius and is being cooled to about 5.5degrees celcius. Boiled off freon vapour is drawn to the suction port at 3.33degrees celcius at 5.2kg/cm2G.

How does this evaporation cooling works ? Whats the difference between boiling and evaporation ? the freon temperature is high when it enters the evaporator and together with the glycol, the freon temperature should increase right ? since there is so much heat.

When the freon boil the temperature should be the same as the liquid ? I'm so confused. Water is heated, it boils i don't see the temperature of the water is reduced.

Edited by newtech, 25 November 2010 - 08:36 AM.

[Art Montemayor]

Newtech:

Your thinking and logic is convoluted and mixed up. A refrigeration system works much more simpler than what you think. You are getting confused because you fail to generate the vital and essential means to understand and communicate engineering information: a simple schematic flow diagram.

If I am to continue to respond and try to help you out on this important application, I will have to generate the simple schematic flow diagram – since you haven’t done so (or won’t). Otherwise it is almost impossible to help you understand how the system works.

That means, of course, that I do all your work for you on this matter. I continue to harp to all students and young engineers how important it is to furnish ALL basic data. That includes accurate, detailed simple schematic diagrams. But by not following my advice, they are continuing to make an engineering career more difficult and prone to errors – something that you will finally wind up paying for in the future. It becomes exceedingly difficult to obtain help when a student doesn’t assist or become the most interested partner in the process of obtaining engineering help – especially free help.

It is also apparent that you haven’t taken the time and effort to use the SEARCH machine in our forums to find information and answers to your questions. Had you done so, you would find a lot of information on refrigeration systems and cycles as well as on pool boiling (or evaporation) systems.

In the attached workbook you will find your answers and a very detailed and self-explanatory flow diagram that you can use to learn what is happening in a refrigeration cycle. With this diagram you can also generate more important queries. Feel free to resort to using this type of enginering tool to help you out in dominating this subject matter.

I have edited and formatted your original post to read as follows below. Note how I follow the simple English rules of grammar – such as spacing between sentences, using complete sentences with definite articles, paragraphs, and proper spelling (use spell check!).

"Hi:

This is a industrial refrigeration system that uses a flooded evaporator and accumulator. Glycol is in the tube bundle and the shell side contains Freon R22. The Freon temperature is at about 45 degrees celsius and 17.64 kg/cm²A as it flows from the condenser, through a filter dryer, and then through a level control valve and into the evaporator shell side.

Glycol enters the tube bundle at 12.8 degree celcius and is cooled to about 5.5 degrees celsius. The boiled off Freon vapor is drawn from the evaporator and into the suction port of a mechanical compressor at 3.33 degrees celsius and 5.2 kg/cm²G.

How does this evaporation cooling work? What's the difference between boiling and evaporation? The Freon temperature is high when it enters the evaporator and together with the glycol, the freon temperature should increase since there is so much heat - right?

When the Freon boils, should the generated vapor temperature be the same as the liquid? I'm so confused. When water is heated, it boils and I don't see the temperature of the water being reduced."

Attached Files

•  A Water Chiller Tutorial.xls   319KB   74 downloads

[newtech]

Hi,

Thanks Art for investing your precious time by helping me. But I'm still having trouble understanding.


At atmosphere conditions, water evaporates because it takes in heat from the surrounding. Which cools the surrounding. Now, liquid Freon(R22) at 45degC and 17kg/cm2, at this point it is unable to be in gaseous state
due to the high pressure.

This warm liquid Freon then fed to the evaporator, which is a low pressure area. So the pressure drops and the Freon should vaporize right ? As it has enough heat to vaporize, which actually mean the entire liquid Freon should vapourize ?


Thanks

Attached Files

•  flooded evaporator.ppt   212KB   53 downloads

[Art Montemayor]

Newtech:

Thank you for the effort and detailed quality of your submitted sketch on PowerPoint. Your product communicates a 100% better than your previous attempt – and it is appreciated.

I can furnish direct and specific answers to your queries in the following manner, with my replies in RED fonts:

At atmosphere conditions, water evaporates because it takes in heat from the surrounding. Which cools the surrounding.
NO! Water does NOT “evaporate because it (solely) takes in heat from the surrounding”. It evaporates in accordance with Mr. Dalton’s Law – it conforms to issuing a vapor pressure, at its temperature, such that it equates to the atmospheric temperature together with the other components that make up the atmosphere (including water humidity). If the humidity content of the atmosphere is low and the temperature of the liquid water relatively high, then the liquid water will exude a vapor pressure such that it will contribute to trying to saturate the atmosphere. In doing this, the liquid water evaporates and causes a vaporization action that cools the mass of liquid water. This is the exact effect that takes place in an atmospheric water cooling tower. This is also the way that our human bodies cool off in order to control the body temperature of our blood circulation system: by producting surface perspiration on our bodies’ skins that tends to vaporize into the atmosphere and produce a cooling effect on the surface of our skin. It is all about Partial Pressures and what Mr. Dalton predicted.

Now, liquid Freon (R22) at 45 degC and 17 kg/cm2; at this point, it is unable to be in gaseous state due to the high pressure.
NO. Refrigerant R-22 is in the liquid state because that is the determined thermodynamic phase that it belong to when it finds itself below its critical temperature (96.145 ^oC) and within the liquid phase dome defined by its thermodynamic properties. In fact, the R-22 can exist as saturated liquid OR SATURATED VAPOR at the stated conditions of pressure and temperature – or it can exist as a mixture of both with a “quality” of liquid mixed in with the saturated vapor. Refer to the R-22 Mollier Diagram or to the thermodynamic Data Base.

This warm liquid Freon then fed to the evaporator, which is a low pressure area. So the pressure drops and the Freon should vaporize right?
WRONG! The “liquid Freon then fed to the evaporator” goes through a throttling, expansion valve. This valve forces an adiabatic free expansion which is considered as irreversible and consequently establishes an adiabatic isenthalpic process (Delta H = 0). This adiabatic process causes a rapid vaporization resulting in cooling and a saturated liquid and vapor mixture of R-22 entering the evaporator. The liquid portion contributes its latent heat to the heat transfer in the evaporator and the resultant saturated vapor produced is what is recycled back to the refrigerant compressor. Only a small fraction of R-22 is produced as saturated vapor in the expansion valve. The produced cold liquid is what yields the refrigerating effect through the contribution of its Latent Heat. THAT is how the R-22 is vaporized within the evaporator.

As it has enough heat to vaporize, which actually mean the entire liquid Freon should vapourize ?
The “entire liquid Freon” DOES NOT VAPORIZE! Only that portion of the R-22 refrigerant produced as saturated liquid by the adiabatic expansion valve is the quantity that contributes LATENT HEAT to the incoming (glycol) heat sink. This is effect takes place by controlling the R-22 liquid level within the evaporator with a level controller that actuates the expansion valve as required.

I hope this detailed expanation answers all your queries and doubts. Please let me know if I have succeeded in thoroughly explaining all of your queries and doubts. I am appreciative of you genration and contribution of the PowerPoint exhibit because it represents a portion of your vested contribution towards what will eventually be your profitable benefit from this thread.

[newtech]

Hi:

But Art the control valve before the evaporator is just a normal level control valve. In the tube side of the evaporator is glycol entering at 12.8degC and is being cooled to 5.5 degC. So it means the heat from the glycol is being transfered to the liquid freon and it evaporates and cools the rest of the remaining liquid freon inside. Lets say the glycol return back to the evaporator at about 5.5degC, which means lesser heat or no is being transfered to the liquid freon. The freon inside should boil by it self as it has reach the boiling point for that pressure? Pressure drop, temperature will also drop, but the amount of heat energy inside is the same.

Alcohol evaporates and produce a cooling effect, just like it touches your skin, it takes heat from your skin and evaporates, thats why you feel cold. If a 45degC alcohol poured on your skin, would you still feel the coolness or would you feel a 45degC alcohol?

I think i am lost too deep, to be found back.

As i do not understand 45degC at 17kg/cm2, pressure drop to 5.2kgcm2 it should vaporize immediately as it is 45degC, at that low pressure its boiling is lower. But yet it has so much heat, so all of the liquid has enough energy to flash.

[Art Montemayor]

Newtech:

You continue to be confused for a variety of reasons. One reason is that I was unable to make comments and notes on your PowerPoint sketch. That is why I always recommend that members use an Excel spreadsheet to make their sketches if they want comments and recommendations made on them. That is a quick and accurate way to exactly indicate where the comment is made and why.

Another reason seems to be because you don’t have the proper (or enough) learning in refrigeration thermodynamics. I have seen our Forums change recently and noted that the Personal Background feature was removed. This, in my opinion, is bad when dealing with students. I am unable to tell or know what level of learning or experience you have or where you are located. The more personal background I have, the better I can tailor my responses so that the member can better understand what I am trying to convey. From what you write I get the impression that you don’t have a thorough grasp of what is happening in a normal, conventional mechanical refrigeration cycle. Have you read and studied the Chiller Tutorial Workbook that I uploaded for you? Have you gone over the sample calculations I did? Did you study and understand what is shown in the process flow diagram I drew? Did you fully understand what is happening in each of the different components within the refrigeration cycle? Do you completely understand the concept of phase equilibria and the importance of distinguishing and establishing different fluid PHASES within the same system? Have you plotted your refrigeration system and cycle on the Mollier (or T-S) diagram?

If you haven’t done the above, then that could explain why you are having difficulties understanding what is happening. You return with additional (actually, the same questions – but with a different slant) queries and my responses are – again, in RED – as follows:

But Art the control valve before the evaporator is just a normal level control valve.
This is true. However, the type or make of the valve means nothing from a thermodynamic point of view. What is important is that the relatively higher pressure liquid refrigerant (which is saturated) enters the evaporator on demand by the liquid level in the same evaporator. Once it enters the evaporator, it is subjected to a lower pressure – which is part of the driving force that allowed it to enter. This action constitutes what is called in thermodynamics as an irreversible, adiabatic expansion (defined as a process where the enthalpy is held constant). If you use your Mollier Diagram (or T-S chart) you will see that the constant enthalpy expansion from 17.64 to 5.2 kg/cm2g will yield a R-22 mixture within the “dome” that defines the saturated liquid and vapor lines. The “normal level control valve” (the expansion valve) merely allows the HP R-22 to enter the evaporator; the action of the R-22 compressor is what is maintaining a constant, 5.5 kg/cm2 pressure in the evaporator. The valve does NOT CREATE refrigeration. It is the maintenance of the evaporator pressure by the compressor which allows for the fluid expansion to take place. The valve is merely a tool to maintain the constant liquid level. It is important that you tell me if you know and understand what I just stated. If you don’t, we can’t continue in trying to show you how mechanical refrigeration works from an engineering point of view.

In the tube side of the evaporator is glycol entering at 12.8 oC and is being cooled to 5.5 oC. So it means the heat from the glycol is being transfered to the liquid freon and it evaporates and cools the rest of the remaining liquid freon inside.
You are correct in stating that there is heat transferred from the glycol to the liquid Freon. HOWEVER, the evaporation of the Freon due to its receiving Latent Heat DOES NOT cause further cooling of the remaining Freon in the evaporator. As explained above, it is the compressor that causes the refrigeration cycle to operate due to its removal of the saturated vapor produced by the latent heat added by the glycol. REMEMBER: the evaporator has SATURATED Freon liquid and vapor, so the liquid Freon is subject to being vaporized by the addition of latent heat.

Lets say the glycol return back to the evaporator at about 5.5 oC, which means lesser heat or no is being transfered to the liquid freon. The freon inside should boil by it self as it has reach the boiling point for that pressure? Pressure drop, temperature will also drop, but the amount of heat energy inside is the same.
If the glycol enters at a temperature close to that of the liquid R-22, then the heat transferred will diminish or stop. When that happens, the compressor will suck less R-22 vapors (since there is less – or no - vapor being produced) in order to keep the vapor pressure in the evaporator at the set point of 5.2 kg/cm2g. You are producing less refrigeration because it is not required. The liquid Freon does not evaporate anymore. It just sits there – still in the saturated state – waiting to receive latent heat from the warm glycol. (You continue to worry me by not specifically identifying the state and phase of your fluids. If you are describing the SATURATED LIQUID Freon inside the evaporator, then state is as such. DO NOT describe it as “freon inside should boil”. This is probably one of the reasons for your confusion.

Alcohol evaporates and produce a cooling effect, just like it touches your skin, it takes heat from your skin and evaporates, thats why you feel cold.
Correct. This is the effect that you detect when you apply alcohol on your skin and the heat transfer mechanism that makes it work.

If a 45 oC alcohol poured on your skin, would you still feel the coolness or would you feel a 45 oC alcohol?
Yes, you would. Stop and apply the previous correct statement you made. Even though the alcohol is much warmer than your body and there can be no heat transferred from your body to the alcohol, the alcohol is still subject to Mr. Dalton’s Law. It will exert a vapor pressure to fulfill the need to have all the Partial Pressures equal to atmospheric. When that happens, the alcohol is vaporizing – because it is “expanding” (actually, vaporizing & cooling) into a vapor phase due the action of partial pressures. The dryer the atmospheric air is, the faster the evaporation – and the subsequent cooling of the film of alcohol remaining on the skin surface. It is the driving force of partial pressures that allows the alcohol to evaporate and cool. The moment all the liquid alcohol on the skin surface is evaporated, the cooling ceases and you have to apply more liquid in order to continue the effect. Note that once the alcohol starts to cool due to partial pressure evaporation, there will follow a normal heat transfer from your skin surface to the cooled liquid alcohol. This then raises the temperature of the residual alcohol and also raises the partial pressure of the alcohol – so the evaporation process continues (but only as long as the atmosphere can take the contributing partial pressure addition of alcohol vapors).

I think i am lost too deep, to be found back.
Don’t give up trying. If you do, you will force me to stop helping you.

As i do not understand 45 oC at 17 kg/cm2, pressure drop to 5.2 kg/cm2 it should vaporize immediately as it is 45 oC, at that low pressure its boiling is lower. But yet it has so much heat, so all of the liquid has enough energy to flash.
Read the above paragraph and tell me what fluid you are writing about. As I stated before, you are not writing clearly and therefore communicating very badly. Please correct this defect for your own good. If you are referring to the HP liquid R-22, then state so. I believe that you are and I have explained already what is happening in the “expansion” of this fluid. But I will repeat it again (because it such an important point): The HP R-22 liquid fluid entering the “expansion” valve is in the SATURATED state. If you locate it on a Mollier Diagram and follow it along its constant enthalpy line to a pressure of 5.3 kg/cm2g, you will discover that you are inside the phase “dome” of the diagram. This dome defines the CRITICAL POINT and the saturated liquid (left-hand side) and vapor (right-hand side) phases. You should already have been taught this material and should know how to employ it to find data and to make calculations.

I hope that the above detailed explanations help you understand and also to communicate accurately and correctly.