View Full Version : about the refrigerant fill amount

hammurabi

14-07-2006, 07:28 AM

hi sirs:

I meet a formula to calculate the refrigerant fill amount of flooded chiller's heat-exchanger like this :

R.charge = b1 + b2*shell's inner diameter + b3*shell's inner diameter*shell's inner diameter + b4*shell's inner diameter*shell's inner diameter*shell's inner diameter

what's this formula meaning?

and how to calculate the b1/b2/b3/b4?

anyone can commend some literature about the refrigerant fill amount of flooded chiller ?

thanks for all u

shylockw

14-07-2006, 03:25 PM

hi sirs:

I meet a formula to calculate the refrigerant fill amount of flooded chiller's heat-exchanger like this :

R.charge = b1 + b2*shell's inner diameter + b3*shell's inner diameter*shell's inner diameter + b4*shell's inner diameter*shell's inner diameter*shell's inner diameter

what's this formula meaning?

and how to calculate the b1/b2/b3/b4?

anyone can commend some literature about the refrigerant fill amount of flooded chiller ?

thanks for all u

That is only an empirical equation. b1/b2/b3/b4 are the fitting coefficient for the polynomial function. Usually, researcher will fit a polynomial function based on the experiment data to interpolate and extrapolate the value.

you can consult the person who give you this equation (only empirical).

US Iceman

14-07-2006, 03:54 PM

If this is for a flooded shell & tube chiller the equation looks like it based on the shell inside diameter with "b1/b2/b3/b4" being variables. Based on this assumption the variables would have to be:

Tube outside diameter

Shell length

tube pitch

Operating liquid level in the shellThat's quite a bit of calculations for a simple curve fit.

anyone can commend some literature about the refrigerant fill amount of flooded chiller?

Depending on the shell configuration you could have an operating level of 50-75% of the inside shell diameter. Some HVAC flooded chillers have the inside shell only half full of tubes. Others may have a full tube bundle covering 100% of the inside diameter with tubes.

hammurabi

17-07-2006, 02:45 AM

thanks for skylockw and US Iceman's warm reply

the fitting coefficients come from our USA R&D Dept.

so it's hard for me to know how it comes from.

US Iceman：our chillers always have an operationg level of 50% .I havn't seen 100% level of inside diameter flooded chillers,if this ,can't the liquid go into compressor easier?what size of spare space is best? 50%?60% or 75%? It really bother me.

IF u have any literatures about refrigerant chargre of flooded chiller, pls do not hesitate to commend to me .

thanks again

best wishes

regard

hammurabi

shylockw

17-07-2006, 11:40 AM

the fitting coefficients come from our USA R&D Dept.so it's hard for me to know how it comes from.

Are you making project budget or tender? If you dont get any fitting coefficient, I suggest you have no hesitation to estimate value by yourself.

The filling refrigerant is mainly full in evaporaor(m1), piping line(m2) and condensor(m3).

m1= 0.25*3.14*[(evaporator inner diameter)^2]*evaporator tube length*full rate %*refrigerant density.

m2=0.25*3.14*[(Piping line inner diameter)^2]*Piping line length*refrigerant density.

m3=0.25*3.14*[(condensor inner diameter)^2]*tube length*full rate %*refrigerant density.

I think you can estimate it.

US Iceman

17-07-2006, 03:42 PM

US Iceman, our chillers always have an operating level of 50%. I haven't seen 100% level of inside diameter flooded chillers...

The liquid level is not at 100% of the shell diameter. The tube bundle is. Please refer to the attached sketch.

Your bundle is on the left. This is referred to as a kettle-type chiller. The tube bundle does not completely occupy the inside shell diameter. The liquid level is maintained somewhere below the top row of tubes.

As the boiling occurs, the liquid level will rise. The upper portion of the inside diameter does not have tubes, but usually does have some type of liquid separation device (demister pads, chevron packing, or other) to help in the removal of liquid droplets from the gas flowing up to the compressor.

The other picture on the right side of the sketch is a full tube bundle. More tubes in the same size shell, so you have more cooling capacity.

The liquid separation is accomplished by the external vessel on top of the chiller. The liquid and vapor rise up through the pipes connecting the top vessel to the chiller below. The connecting pipes are called risers.

The vessel on top is normally called a "surge drum".

For the picture on the right, the liquid level is normally controlled at the ~75% of the inside shell diameter. Again, the liquid level will rise due to the boiling, so the top tubes will be wet.

They also help to dry out the gas flowing upwards.

hammurabi

18-07-2006, 07:04 AM

thanks for your two's warm reply.

from skylockw's words,I can calculate the flooded evaperator's charge amount ,by using 50% filling rate and refrigerant at 5 degree(design saturating vaporization temperature). But that refrigerant charge amount in condensor still confuses me .Because I just know the design saturating condensing temperature 37 degree.The refrigerant state in condensor is mixture of vapor and liquid.what temperature and pressure should I choose to calculate the density.Though the tubes have filled fully in the condensor shell, the filling rate can't be 100%,I think.Should I calculate by 50% saturating liquid and 50% saturating gas?

It's the first time for me to have seen this type of flooded evaporator.thank US Iceman for your drawing.

shylockw

18-07-2006, 02:55 PM

Though the tubes have filled fully in the condensor shell, the filling rate can't be 100%,I think.Should I calculate by 50% saturating liquid and 50% saturating gas?.

Are you working for Carrier or Trane?:)

I think the filling rate should be more than 50% in the condensor. Usually,the subcooling and more small TD will be selected for the condensor selection. On the other hand, the most refrigerant will be liquid in the winter running.I had ever use 65%, It's only my own estimate. (Only for reference):D

Hello Us Iceman, could you give some suggest??:p

If condensing temp. is 37 degree, i think this is a evaportive condensor. so you can find the suggest filling data from the evaporative condenser's manual.

Good luck for you.;)

US Iceman

18-07-2006, 04:37 PM

The way I do refrigerant charge calculations is based on volume & refrigerant density.

A flooded chiller is similar to a shell & tube condenser. The refrigerant is on the shell side (outside of the tubes).

1) Determine the total volume of the tubes,

2) Subtract this from the total volume of the inside of the shell,

3) The remainder is the volume of the space the refrigerant will occupy.

Now, we have to determine what percentage of this volume will contain liquid or vapor.

If your chiller has a 50% liquid level during normal operation, then 50% of the volume calculated in #3 above will be liquid. The remaining 50% of the volume of #3 above will be vapor.

Find the density of the gas and liquid at the evaporating temperature.

Multiply the 50% volume of #3 times the liquid density, and 50% of #3 times the gas density. The total refrigerant charge is the total mass determined from above.

For condensers the process is similar, except the percentage of liquid and vapor is different. If the condenser was designed to provide subcooling, then there will be more liquid inside of the shell.

A reasonable estimate would be to use 80% of the volume of #3 for the gas occupying volume and 20% of the volume of #3 for the liquid containing volume.

Remember these are called estimates for a reason.;)

hammurabi

19-07-2006, 04:22 AM

thanks for US Iceman and skylockw's warm reply.

I am working at McQuay,designing single screw flooded chillers(R22/R134a/R410A).more contact,pls add my msn :huwade@hotmail.com

I just use US Iceman' method to calculate our chillers' refrigerant charge amount.The method gives me satisfying results(15kg more than ours,maybe I should low the filling rate of evaporator).thanks again.I think the 80%gas/20%liquid can use in most application.It's really useful.

Though b1/b2/b3/b4 still confuses me ,I have studied the simpler method to calculate the refrigerant charge amount from ur teaching.To get the best charge amount,we have to use test, I think.

US Iceman

19-07-2006, 02:59 PM

hammurabi,

Remember there are two things to be aware of. The refrigerant level at operating condition, and operating level with the unit shut down.

When the unit is off, the liquid level in the chiller may be lower. When the compressor is at 100% of load more boiling is occurring in the chiller. The boiling will raise the liquid level slightly.

The operating charge calculated with the b2/b3 etc variables may be based on a slightly lower liquid level to account for this.

If you reduce the available liquid volume to ~45%, and use 55% for the gas, this may bring you closer to the numbers calculated by the factory equation.

Conversely, the 80/20 rule may require revision to maybe 90/10 depending on how much subcooling is required to be done in the condenser.

hammurabi

31-07-2006, 07:02 AM

thanks US Iceman ,agree with u !

the max refrigerant fill amount ,we calculate like this :

85% * liquid density (30 degree,saturated liquid ) * free volume of condenser

anther questions :

1.what's the min gas volume in evaporator we should consider ?

2.any good ideas to let the lost oil in evaporator come back ??

happy to learn from u

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