Hi guys,
Could anybody explain me the difference between gravity liquid/vapor separator and knockout drum :confused:
Thanks for all,
Volnei
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Hi guys,
Could anybody explain me the difference between gravity liquid/vapor separator and knockout drum :confused:
Thanks for all,
Volnei
Hi Volnei,
I could be wrong' but from what I've read in the past 20 minuts, knock out drums do not belong to refrigeration or AC.
Chemi :)
Dear Chemi,
First of all thanks for your replay.
Please take a look at the file attached taken by ASHRAE Handbook 1998.
Regards,
Volnei
No difference. In your attached ASHRAE reference, the terms are used interchangeably. The discussion compares light to heavy entrainment designs, but either term is acceptable for either, though knockout drum is more common.Quote:
Originally Posted by Volnei
Rog
Hi Guys
Aliquid knock out pot or drum as you refered to is normally placed in the suction line of large ref plants to stop liquid being pulled back into the compressors which usually happens when the the system comes of defrost or has a rapid change in load ie the load suddenly drops off and the evap starts to flood before the hp float or expansion device has time to close off.
A gravity pot/drum just means it is placed in a position as to allow the liq to run into the vessel by gravity and normally along side the compressor/s, nh/3 systems normally.
Have a nice day'
tony.
Hi Guys,
Mark Fiddy will probably be able to give you a technical term for this that i am about to call a knockout pot.
Large Supermarkets, Large pressure vessell under the rooftop condenser and when the presure gets to high then the gas get diverted into this reciever type vessel. In theroy the gas should then get drawn back into the system but it doesn't always happen so you have to shut the condenser off and let the suction pull the gas out of it.
Fairly sure it's on Searle condensers and if you are reading Mark the one thats sprongs to mind is the store at Shefford.
Regards
Raymond
Hi Guys
There seems to be some confusion here in regard to the gravity liq/vapuor drum come knokout pot. A gravity liq/vapour drum is the same thing as a knockout pot and is found in the suction line of larger ref systems, its purpose is to stop liquid flood back into the compressor, the saturated liquid/vapour mixture enters the pot in the top or side of the vessel the inlet line goes down to the bottom of the vessel and the outlet comes off the top of the vessel making it impossible to pull liquid back into the compressor, the is normally a small dip tube which is fixed into the outlet pipe and goe,s down to the bottom of the vessel to skaving the oil back into the compressor which happens on low load.
The vessel refered to under the condenser is a liquid reciever no matter how you look at it, it,s still a liquid reciever, while on the point of liquid recievers if any one has a liquid reciever situated in the plant room with the condenser mountd above on the roof there is a fernominum that can occur known as liquid hang up in the condenser due to the reciever being mounted in the warm plant room and the condenser mounted out in the cold ambient temperatures, what happens is the liquid will not fall into the reciever because the liquid/gas in the reciever is at a higher temperature than the liquid waiting to fall down down the liquid condensate line. any of you that may have been plauged by call outs normally at night and on attending site find all is ok except that the liquid level alarm had triggered and reset may well pay attension to this fernominum, the fix is so simple, run a 1/4' gas line from the lead compressor discharge line off the vale port or about 500mm from the discharge valve to the top of the liquid reciever, this acts as a gas balance line and allows the liquid in the condenser and liq condensate line to fall back into the reciever, just a little gem of info for the poor sods on call out.
Can the guy who first placed the post inform us all of exactley where this drum/pot is situated ? then maybe we can assist further.
Have a nice day !.
Tony L.
Dear Tony,
I´m not using a knockout drum. I´m just trying to understand what the term "knockout drum" means because I´m writing a procedure about how to calculate liquid separator vessels and in the ASHRAE Refrigeration Handbook Chapter 1 this term appears. Please take a look at the attached file that I sent in a before replay.
Here in Brazil we usually use the term Liquid Separator.
Thanks for all.
Volnei :)
[QUOTE=Volnei]Hi guys,
Could anybody explain me the difference between gravity liquid/vapor separator and knockout drum :confused:
Thanks for all,
This may be of some help :D
http://www.amistco.com/BULLS_PDF/SucDrums.pdf
I may well be wrong, and probably am but, I get the impression that a knockout drum is designed to change the velocity of the moving liquid/gas mixture to encourage liquid separation more than in a normal accumulator.
Ray,are you talking about the Enviroguard packs.(Expansion tank) :eek: :)
http://fridgetech.com/miscellany/enviroguard/
Hi :)
my understanding is that a knock out or slop pot is a simple device designed to stop the occasional liquid carry over from entering the compressor, a suction accumulator by it's posh name. A separtor vessel or gravity liquid vessel would carry out various other functions such as liquid level control and in the case of a pumped system provide a proper head of liquid to the pumps.
Kind Regards. Andy.
So the knockout drum could probably be a accumulator :cool:Quote:
Originally Posted by TONY LONGTHORNE
The formulas posed are Stokes equation and is from the ASHRAE Handbook. It is also found in several other text and handbooks.
The term g/l separator, suction pot, suction drum, suction accumulator, KO pot, KO drum are all used pretty much to describe the same device which is a large liquid trapping area in the suction of a refrigeration system. The smaller equipment Will often use an accumulator with a "pick-up" with a metering orifice to return oil and refrigerant to the compressor at a controlled rate. They will sometimes have coil in them for either subcooling of the HP liquid or a smaller coil called a "boil-out" coil. he boil out coil will aid in vaporizing a liquid slug to dissipate the liquid faster.
A large system will often use an accumulator with a boil out coil in conjunction with a oil still/pot to return oil to the compressor.
Most larger refrigeration vessel suppliers have catalogs with engineered accumulators and sizing charts. Might look at places like Chilcon, Standard Refrigeration and Refrigeration Research. For the larger sizes I have develops a series of graphs to determine separation velocity and separation distances.
The term g/l separator generally applies to most refrigeration accumultors which do not use demister pads or impingment vanes. A typical accumulator is depending on velocity reduction, change of direction, vertical separation distance and gravity for proper separation of the liquid and vapor. Other separators depend on a mesh pad, vanes or centrifugal force to start coalescing of te droplets to form larger and larger droples to drop out.
Ken
Sounds to me like a Surge Drum used in NH3 flooded evap systems. :confused:
Liquid pumped from surge to evap and wet-suction returned to surge. Vapour pulled off surge thus reducing press./temp.
Sounds to me like a liquid over-feed drum or a recirculator which are often used in ammonia systems. The sizing criteria for this application is exactly the same as a suction accumulator. You have to have sufficient distance and velocity reduction to allow the liquid to drop out and provide a clear suction to the compressor(s).
What exactly are you trying to accompolish with your design?
Ken
Volnei:
As some responses have already told you, there is no difference between a vapor-liquid separator, a knockout drum, a flash drum, or compression suction drum, etc.
How To Design A Vapor-Liquid Separator:
A vapor-liquid separator drum is a vertical vessel into which a liquid and vapor mixture (or a flashing liquid) is fed and wherein the liquid is separated by gravity, falls to the bottom of the vessel, and is withdrawn. The vapor travels upward at a design velocity which minimizes the entrainment of any liquid droplets in the vapor as it exits the top of the vessel.
The size a vapor-liquid separator drum (or knockout pot, or flash drum, or compressor suction drum) should be dictated by the anticipated flow rate of vapor and liquid from the drum. The following sizing methodology is based on the assumption that those flow rates are known.
Use a vertical pressure vessel with a length-to-diameter ratio of about 3 to 4, and size the vessel to provide about 5 minutes of liquid inventory between the normal liquid level and the bottom of the vessel (with the normal liquid level being at about the vessel's half-full level).
For the maximum design vapor velocity (which will set the drum's diameter), use the Souders-Brown equation:
Vmax = (k) [ (dL - dV) / dV ]^0.5
where:
Vmax = maximum design vapor velocity, ft/sec
dL = liquid density, lb/ft3
dV = vapor density, lb/ft3
k = 0.35 (when the drum includes a de-entraining mesh pad)
The drum should have a vapor outlet at the top, liquid outlet at the bottom, and feed inlet at somewhat above the half-full level. At the vapor outlet, provide a de-entraining mesh pad within the drum such that the vapor must pass through that mesh before it can leave the drum. Depending upon how much liquid flow you expect, the liquid outlet line should probably have a level control valve.Quote:
The GPSA Engineering Data Book recommends the following k values for vertical drums with horizontal mesh pads (at the denoted operating pressures):
0 psig: 0.35
300 psig: 0.33
600 psig: 0.30
900 psig: 0.27
1500 psig: 0.21
GPSA Notes:
1. K = 0.35 at 100 psig; subtract 0.01 for every 100 psi above 100 psig
2. Typically use one-half of the above K values for approximate sizing of vertical separators without mesh pads.
3. For compressor suction scrubbers and expander inlet separators, multiply K by 0.7 – 0.8
As for the mechanical design of the drum (i.e., materials of construction, wall thickness, corrosion allowance, etc.), use the same methodology as for any pressure vessel.
The Souders-Brown equation was originally derived from experiments on distillation columns in refineries (about 1933). If I remember correctly the authors stated the use of the "k factors" were based on results seen in the liquid/vapor separation of the columns and may not be applicable to all cases.
The k factors do change with pressure (and vacuum service) and vessel internal design features. In industrial refrigeration you will typically see a k factors between 0.1 to 0.2, depending on the specific vessel construction, and, how much the manufacturer wants to "push" the design.
It should be made clear that the use of k factors higher than 0.2 should be based on the use of separation enhancements such as demister pads, impingement plates, or other devices.
The liquid particle size carried over in the gas stream may also be dependent on the mesh density, wire size, and capacity reduction of the system.
In addition users should also be aware that a demister can flood with liquid. Exceeding the design velocities can also cause larger liquid slugs to exit the vessel, since the demister pad can hold liquid in suspension.
Other types of separator enhancements besides demister pads are available, but in the refrigeration industry gravity separators are used almost exclusively.
Vertical vessels have the advantage on a fixed cross-sectional area, so the separation velocity does not change as the liquid level increases. The separation distance (liquid level to the outlet connection) does however decrease.
Horizontal vessels have variable cross-sectional areas. As the liquid level increases, the maximum allowable velocity should decrease to maintain the desired separation.
Demister pads are not without their own inherent problems. They can foul with debris in the system, cold oil, or have holes punched in them by high velocity debris.
Care should be exercised in the selection, placement, and installation if these are utilized. In process requirements they usually have man-holes in the vessel for internal inspection. This feature is not normally provided in refrigeration, so there is no way to perform a visual inspection on the demister.
As noted in an earlier post, the vessel should accommodate sufficient reserve volume to contain any returning liquid before it reaches the compressor.
This may depend on the number and size of evaporators defrosting, or, events leading to a upset in the operating conditions.
Similar design methods are used for oil separators (this is still liquid separation from a vapor stream). However, in an oil separator we will find coalescing elements to further reduce the oil concentration level (carryover) to minimum levels.
In any event, we should remember all liquid/vapor separators are not 100% efficient.
US Iceman:
Thank you for your comprehensive and useful follow-up to my original post. In regard to this part of your follow-up:
I would just like to point out that the GPSA data on K values that I included in my original post does spell out the effect of pressure and also spells out that the given k values depend upon the use of demister pads.Quote:
The k factors do change with pressure ...... It should be made clear that the use of k factors higher than 0.2 should be based on the use of separation enhancements such as demister pads, impingement plates, or other devices.
HI mbeychok,
I saw the notes. ;)
I only wanted to reinforce the effects of changes in the k factors and the concept for gravity separators. It should also be noted (to other readers of the thread)that most of our application are certainly well below 300 psig and are almost always gravity separation driven.
The k factor concept and method allows for quick development or checking of separator capacities, much more so than using particle sizes, drag, and force balances.
Liquid separation is a fascinating field of study, heh?
Quote:
Originally Posted by TONY LONGTHORNE
hi tony !
is there a specific height that the suction accumulator or knocout pot / surge drum needs to be placed for optimum performance ? i mean should the bottom of the suction accumulator or surge drum be at say on the same height as the top of the evaporator ? or should it be at say the same level as the liquid level ? also, if it is placed lower than the evaporator, what is the best way to drain the liquid back to the evaporator if gravity is not an option ? ,is there a way to dry it off , like say having a sort of a heat exchanger or a vessel with a heating coil ? what is an acceptable liquid level inside the knockout drum ?
thanks
mcr
Geeeeeeeeeez you guys.
Talk about confusing and clouding the initial question.
"gravity vapour separation ".
Remove "knock-out drum " from your minds, that term refers to all and sundry. Gravity separates liquid and specific density of refrigerants dictates the vessel volume/ height to control carry over to compressor.
magoo
It is a bit more to it than you would like to believe Magoo. I have been designing separators ir suction drums or knock out drums fro almost 40 years and they are not designed like the process fellow design.
First thing is the process group designs for a certain liquid entrainment as being acceptable in normal operation. They will usually go with some like 2% of the volume or droplets under a certain size. While acceptable in a process operation, this is not acceptable in refrigeration system where you need a dry suction.
Another items comes into play here and you have to consider surface tension as well as the relative densities of the vapor and liquid. Dsiengaging height is also a big consideration for all suction separators or flooded evaporators. I have used Stokes equation and use 80% of the calculated max velocity at the highest suction temperature you expect to operate during operation and look at start up conditions as well. I have never had a suction drum carry liquid to the compressor during operation. Yes my designs are a bit larger than competitors, but The customer does not destroy compressors. Larger suction drums are cheaper than compressors.
For refrigeration applications, I have never liked demister designs. As Iceman noted, once you get them too wet or plugged, things change and they do not work so well. Per ASME codes, the refrigeration vessels are non-corrosive service and do not require manways, so the vessels are welded closed and no way to get to the demisters unless you cut the vessel open.
Evaporator and suction trap design is something that you can spend hours on as a class.
ken
I can see this turning into a long discussion.;)