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I'd like to know how to make the sizing for an equalising line for hot gas from liquid receiver to condenser or evaporative condenser.
Fluid is saturated vapor, but:
1) how to know the rate kg/h? (the total rate of the whole system?)
2) Wich overpressure from liquid receiver do I have to 3 suppose? delta P =1 bar is enough? or to big?
3)1 m/s is a goog value for speed in this line?
Thanks a lot
davidlleida
You have just asked a question that will generate a lot of replies, I think.
Think about the process of the equalizer line. It allows the gas in the receiver to equalize with the gas in the condenser inlet. If the receiver pressure is higher than the condenser outlet pressure (drain line) for any reason, liquid will hang up in the condenser (not good!). To prevent this the equalizing line must equalize the receiver pressure and condenser inlet pressure.
The individual P-Traps in the separate condenser drain connections provides static pressure to equalize the drain line pressure with the receiver pressure.
If the equalizing line cannot equalize the pressure of the receiver the receiver pressure increases. Here is one way to look at this. As a cubic foot (or meter) of liquid flows into the receiver, the same volume of gas must be vented to the condenser inlet. Otherwise the receiver pressure increases.
I size this pipe with an extremely low pressure loss, such as less than 0.5 psi (0.03 bar)! The amount of gas the equalizing line must vent is: volume of liquid flowing into the receiver (this is the volume of gas that must be vented) + the flash gas created if liquid boils in the receiver. Remember, the liquid in the receiver is saturated. If the ambient temperature is greater than the saturated liquid temperature, the liquid will boil off and create additional gas that must be vented through the line.
If the equalizing line is sized to big could this cause problems?
My opinion is the equalizing can never be too big. I've worked on some large systems where the equalizing was 12" diameter!
Thank you so much US iceman,
I find your quick answer very helpful and concrete.
Your explanation is easy to understand, now I will think about the correct sizing taking care of the pressure loss.
I need to know the "f" values for saturated vapour, gas and liquid for every refrigerant.
Concerning the idea if nh3wizard, let me ask:
Couldn't it be possible that in some situations, gas exiting the compressor flow directly to liquid receiver passing trough the equalising line? (I understand that liquid pipe size is correct and so the design)
Once again thank you for your help
I've been sometimes afraid of theese
Quote:
Originally Posted by US Iceman
I have seen design engineers install the equalize line and on start up come to find out it was to small and have to have the size increased, why not over size it slightly to begin with.
This is the problem as I see it. People have have treated these lines as;Quote:
Originally Posted by nh3wizard
a) not required
b) required but installed too small because this is most common, or
c) the vessel manufacture provided a 3/4" connection, so that is what size that will be installed (after all, the manufacturer knows what he is doing, right?);)
It is so easy to do this when the vessel is being purchased, but if done after the fact, the vessel connection is too small.
Condenser piping, drain line traps, and equalizing lines have to be the most misunderstood topics in industrial refrigeration...
Here is a topic that will generate some discussion. Why don't we insulate high pressure receivers?
davidlleida,
Equalizing lines can have gas flow in either direction. That is their purpose.
I have never been given a definitive answer on why high pressure recievers do not get insulated; not required; because its at high pressure... some of the reasons I remember... So why dont they get insulated?
It's really not a problem to oversize this line?
gas from compressor could not reach the receiver passing through this line?? (when pressure drop where lower than condenser and liquid lines)
Some books give speed values for other lines, wich makes sizing in not too long lines not so difficult.
These would be a good way to size this one?
Are there other easy methods wthout having to do the calculation of "f", then pressure drop, and finally D?
Thank you,
Quote:
Originally Posted by nh3wizard
Think about a high pressure liquid line.... The liquid is essentially saturated in the receiver, therefore if the condensing temperature is 95°F and the ambient temperature (surrounding the receiver--- this could also be the engine room temperature!) is higher than the liquid temperature, what happens?
The liquid boils off into a vapor. This vapor must be vented out of the receiver by the equalizing line otherwise the receiver pressure begins to increase.
If you insulate the receiver, the amount of heat leaking into the liquid refrigerant is much less which means there will be less vapor volume generated that the equalizing line would have to vent.
This also applies to high pressure liquid lines that are not subcooled. Any ambient temperature (or solar induced temperature) that is higher than the saturation temperature of the liquid in the pipe will flash off, which creates all sorts of problems with liquid feed lines.
Quote:
Originally Posted by davidlleida
If the line is too big, what could happen?
Vapor velocity for pipe sizing is OK as long as you know the pressure loss. Condenser piping is like a manometer. You are trying to balance pressures to prevent the liquid from stacking up in the condenser coil.
I much prefer to calculate the Reynolds number, friction factor, etc to find the calculated pressure loss. If the receiver pressure is too high, the condenser will not drain. You would not know what the pressure is without doing pressure loss calculations.
You have to be very careful with some of the values used for equalizing line sizes in charts and tables. Most of this is based off of old data that may not be acceptable.
If you use someones value and are wrong, who is at fault?
"Why don't we insulate high pressure receivers?"
1) would the insulation be enough to avoid that pressure in the receiver gets higher than in condenser?
2) insulation costs of a receiver can be high
3) Please accept my apologies if I'm asking siily things, but what does my sistem win with flow of gas from compressor to receiver by equalising line? that way we also add difficult to the liquid to reach the receiver isn't it?
the more I think about I find the correct size of this line is more important
Whe we have had some problems in this line we close a little a valve in that line to avoid gas from compressor reach the receiver
Thank you all for your comments, I'll be waiting for your explanations
"Why don't we insulate high pressure receivers?"
Insulation provides two purposes. Limit heat gain or heat loss.Quote:
Originally Posted by davidlleida
If we can reduce the amount of heat gain into the receiver this will reduce the volume of flash gas that forms. The equalizing line must vent the gas displaced by the liquid flowing into the receiver + the vapor formed in the receiver from heat gain. If the total volume of vapor cannot be vented through the equalizing line at a low enough pressure loss, the pressure in the receiver increases (the gas compresses in the receiver due to lack of flow capability of the vent line).
If the receiver pressure increases, the height of liquid in the condenser increases to balance the pressure loss with static head of liquid.
True, but what is the cost for not designing or installing the system properly?Quote:
Originally Posted by davidlleida
Every time we do the job correctly, it costs money. Every time the job is done poorly, initial costs are saved, but operating costs are higher for a longer period of time!
I do not know how to answer this now because I do not have enough information. What types of problems have you had? The only thing I can think of is that the receiver is condensing a lot of vapor into liquid during cold ambient weather.Quote:
Originally Posted by davidlleida
During low ambient temperatures the refrigerant vapor could flow into the receiver and condense. Remember, the receiver shell in this case may be colder than the saturated temperature of the gas at discharge pressure.
"I do not know how to answer this now because I do not have enough information. What types of problems have you had? "
The "problems" where higher discharge pressure than expected in summer.
Anyway, I should also check in that case liquid lines to be big enough. So the gas would not "prefer" going through equalising line better than trhough condenser could this be a reason??
Finally this is being for me a very helpful post
Once again, thank you so much for sharing your knowledge
OK, but there can a lot of reasons why you had this problem. Were you closing the valve in the equalizing line to help reduce the discharge pressure?Quote:
Originally Posted by davidlleida
If the condenser is sized correctly for the heat rejection of the system and the entering air wet bulb temperature and does not work, then the problem is most likely due to bad piping installation, non-condensable gases, mineral scale deposits, or liquid backing up in the condenser (which could be a piping issue).
The intention was to increase the pressure drop in the equalising line.
liquid lines were sized with low (speed so low pressure drop)
As you say, several reasons could influence the high pressure value.
The true is that actually I can't give much more details, this past summer the system worked well...
I see that I've a lot of work to do with sizing!!!
Ideally you want low velocity (same as low pressure drop) in the equalizing line and liquid drain lines. If the intention was to increase the pressure drop in the equalizing line, then the pressure would have to be higher on one side of the valve (either the receiver side or discharge line side - depending on flow path).Quote:
Originally Posted by davidlleida
I'm not sure how that would help reduce the condensing pressure??
By the increase of pressure drop in equalising line, we have less flow in that line, wich is better for the system.
That flow (in order to equilibrate pressure) will flow through condenser line wich is also better for the system.
Am I wrong?
Isn't it better to force flow go throgh condenser (even by increasing pressure) than allow it to flow through equalising line and arrive to receiver and so, increasing the difficult to liquid to arrive at receiver from condenser?
I disagree. I do not know why you had high discharge pressure but in closing the valve you may have allowed the receiver pressure to decrease just enough to allow the condensers to drain.Quote:
Originally Posted by davidlleida
This sounds like another problem...
You are wrong. Gas flows through equalizing line in opposite directions.Quote:
Originally Posted by davidlleida
For example.
1. When condensing pressure goes up in discharge line, pressure in high pressure receiver will increase through equalizing line. This flow isn't critical, because it doesn't have negative influence on condensers.
2. When condensing pressure goes down in discharge line, pressure in high pressure receiver will decrease through equalizing line. This flow is critical. If we have resistance in equalizing line, liquid won't drain properly form condensers.
Thank you US Iceman and Segei for your help.
About Segei:
1. When condensing pressure goes up in discharge line, pressure in high pressure receiver will increase through equalizing line. This flow isn't critical, because it doesn't have negative influence on condensers
There is never going to be negative effect from Gas pressure in receiver because it's pressure will never be as high as the amount of liquid static pressure from condenser in liquid lines?
But if this it's true, closing the equalising valve should not have any effect on reducing condensing pressure or liquid draining from condenser?
The problem you are asking about depends on where the balance in pressures comes from and when it occurs.
No static pressure in liquid line. Liquid line isn't full of liquid. It means that receiver pressure will be in liquid line up to condenser or up to liquid trap. Condenser pressure +liquid static pressure=receiver pressure. Liquid static pressure apply to bottom of condenser or to the liquid trap. If receiver pressure is higher that condenser pressure, liquid can flood the condenser.Quote:
Originally Posted by davidlleida
This discussion is based on where the traps are actually located under the condenser drain connections (refrigerant). The traps have to be located below the condenser drains by a sufficient distance to generate the static pressure without allowing the liquid to back-up in the condenser coils.
Otherwise, you can still have problems with high discharge pressure.
Thank you Segei for joining in and for your help.
I remember that in the case I told you, liquid lines where not as good we could desire...
Could you tell me what's your usual values about sizing liquid line from condenser through receiver? ( if it's not asking too much...)
1) I choose low speed (near 0,5 m/s), with liquid exit "legs" as vertical and long as possible, but this isn't always made as desired...
2) If liquid line is too big this could be a problem? (It always would depend of a good enough equalising line sizing).
The method I use for evaporative condensers is to install a pipe the same size as the drain line connection. This is typically a 4" pipe. I do not use velocity because the pipe will only have liquid across the full cross-section of the pipe above the trap.
Liquid drop legs as long as possible is a good general answer, but... what is long enough?
The operating conditions change throughout the year so what works in summer may not be enough in winter. Secondly, if you have multiple condenser installed at different elevations you have a more complex problem to solve. One answer for a single condenser is different than an answer for multiple condensers!
The easiest way to sum this up is: the trap height (distance from outlet connection to the top of the trap) is dependent on the worse case pressure loss. Some people use 2-4 feet, some manufacturers will say to double this for winter time operation, while others may say you need 8-12 feet or more.
Generally, more is better but you need to know what you need exactly before the piping is installed.