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Hi
In the case of hot gas defrosting of ammonia evaporators, say for a bootom feed coil, should the hot gas for defrosting come from top or bottom of the coil ?, what is the best arangement and why?
How the above differ if the coil is top feed type?
simon
Hi Simon,
Let's first start out with an explanation of top feed versus bottom feed (it sounds like you already know the difference, but others may be interested).
These terms describe the entrance point of the liquid refrigerant to the evaporator. There is a big discussion that can take place for which is better, but I won't go into that here.
The typical hot gas flow for a bottom feed coil would be: the hot gas initially flows into the coil drain pan. From here the hot gas flows out of the drain pan (coil) through a check valve and into the top of the evaporator coil. For a bottom fed coil this would be the suction connection.
As the hot gas flows into the coil suction connection, it then flows into the evaporator coil circuits. The gas condenses into a liquid. If the hot gas method is using a regulator, the coil is simply pressurized until the coil pressure increases to open the defrost regulator.
Some maufacturers do this slightly different. It depends on how the coil is circuited and how many headers are used. I've seen some where you only have liquid and suction header, others may add a hot gas header, or you may only have two headers (hot gas & suction) and a liquid distributor.
In all cases it is desirable to be able to drain the liquid (the hot gas vapor that is condensed) by gravity out of the evaporator coil.
Hi, :)
this is one way to make hot gas defrosting;) for double sytem cold room
Best regards, Josip :)
Hi Josip,
Hot gas and water defrost... Is this for a spiral freezer, or for a room where you can switch the temperatures?
http://www.pbase.com/kimmo98/image/68110275.jpg
This is one way to do it, with manual defrost.
-SCV 65: Suction stop valve
-SVA 32: Hotgas stop valve
-OFV 25: Hotgas return valve (overflow valve, we set them to open at 5 barg)
-REG 25: Liquid regulation valve
-SCH 25: Liquid non return valve
for defrost, close the suction valve, open the hotgas valve... voila :)
Rotate it 90 degrees to the left and you have an evaporater instead of a plate freezer :)
Suction valve and hotgas valve can be exchanged with solenoid valves of suitable size for automatic defrost.
tho I have never been a big fan of slamming a freezer at 0.1 bar up to 9-12 bar at the flick of a switch, it should be ok in your case, since you are running such high temperature.
A good point to make. Tycho, do you use a pilot solenoid to start a lower hot gas flow, or a regulator to control the defrost pressure (hot gas supply)?Quote:
Originally Posted by Tycho
The ones I've seen are simple on/off systems, suction valve closes as the hotgas valve opens.Quote:
Originally Posted by US Iceman
It's very brutal, this one ship I was on had a plant delivered by Grenco in spain and hadnt the platefreezers been welded to the deck I swear they would have jumped when I switched to defrost, thats the worst one.
Should thing they didnt need the hotgas, but just wanted the shock to shake the frozen fish blocks out of the freezer :)
Some do it in reverse from what I showed in the picture, switch places on the hotgas and hotgas return and then use a 5-10 second delay on the closing of the suction valve. that way, when you turn on the hotgas you will quickly drain a good amount of liquid from the freezer and when the suction valve closes the gas pressure has to build up instead of hitting a liquid obstruction the instant the valve opens.
We havent delivered any systems like this the last 15 years as far as I know, but we did deliver one plant where we used electric actuators (electric motors) on ball valves.
The switch is "freeze - 0 - defrost"
When they switch to defrost, the suction ball valve closes in about 5 seconds at the same time the hotgas valve starts opening and it takes around 15-20 seconds from closed to full open.
Switch to "0", both valves are closed.
Switch to freeze the suction valve starts opening, taking around 30-40 seconds from closed to full open.
the valves open in small steps, it's not smooth, but it's as close as it can get :)
---------------------------------
We use PM valves between the compressor and condenser on 99.9% of our plants, and the hotgas pressure is the same as the condenser pressure, around 7 barg, we also have frequency converters for the condenser water pump on most of our plants, so when a hotgas defrost is started the condenser pressure drops and thus the pump slows down and keeps the pressure steady.
on plants that dont have a frequency converter on the condenser water pump, we suggest using two PM valves to ensure a good supply of hotgas.
Compressor--->-PM-->-hotgas outlet-->-PM--->-Condenser
as an example, the first PM's CVP is set to open at 10 barg and the second is set to open at 10.2 barg.
When a hotgas defrost is started, the pressure drops in the HP line and the second PM closes, routing most of the hotgas to the freezer/evaporator (depending on the compressors capacity at the time of course :))
This ensures a good hotgas pressure, while at the same time keeping the oil in the compressors oilseparator from taking a quick hike out into the plant :D
I think this version would be a LOT safer since it reduces the tendencies for hydraulic shock from the liquid acceleration.Quote:
Originally Posted by Tycho
Of course, doing it the other way would keep everyone on board awake!:D
But doesn't that cut down on the number of times you have to drain oil also?;)Quote:
Originally Posted by Tycho
All joking aside, you make some very good points.
Hydraulic shock, thats the word, or words I was looking for :)Quote:
Originally Posted by US Iceman
since we are allready on the subject, I'll have to add that a plate freezer at -30-35C in a ship factory (water everywhere) gathers quite a bit of ice in the 3-4 hours it takes to freeze a block of fish down to -18-20C.
When a hose with 4 inches of ice, frozen to the hose next to it, is subjected to the shock of going from -0.3 barg to 10 barg faster than you can say "huh?"
Going from a relaxed to full tension like that is going to cause problems, right... right, they start leaking...
also, on factory trawlers, space is not something you have too much of, so the freezers are placed as close to the wall/bulkhead as possible. We use a minimum of 35 cm, that gives us just enough space to stand upright behind the freezer (unless you have breasts :D), and enough place to get back there and change a hose if need be. but the machinists on the ships have no desire to stay back there longer than need be, so when they have a hose burst, they change it, and when they tighten the swivel nut the hose has a tendency of following if you dont lubricate them first...
so after 2-3 months we have to come onboard and change all the hoses as they look like corckscrews after emergency repairs at sea *sigh*
indeed it does :D we try to pound into them that them need to keep a tally of how much oil they fill and how much oil they drain, we even give them log books that are easy to fill out.Quote:
Originally Posted by US Iceman
but it seems like on most ships it goes something like this:
*AAAAAAUUUUUUUUUUUUUUGGGGGGGAAAAAAAAAH*
*Onboard Engineer runs to engine control room*
Onboard Engineer: hmmm "low oil level - ref. plant" *sigh* pumps in 40 liters (just to be sure it'll last the night through)
2 days later:
*AAAAAAUUUUUUUUUUUUUUGGGGGGGAAAAAAAAAH*
*Onboard Engineer runs to engine control room*
Onboard Engineer: hmmm "low oil level - ref. plant" *sigh* pumps in 40 liters (just to be sure it'll last the night through)
rinse and repeat.
Two months later:
-platefreezers starts freezing uneven
-have problems keeping cargo hold temperature
a week later:
*AAAAAAUUUUUUUUUUUUUUGGGGGGGAAAAAAAAAH*
*Onboard Engineer runs to engine control room*
Onboard Engineer: hmmm "low oil level - ref. plant" *sigh* pumps in 40 liters (just to be sure it'll last the night through)
three months later:
has to abort fishing and go in to get a service engineer onboard to drain 200 liters of oil and have said service engineer point to drum full of oil and say "now there's your problem"
the first thing said service engineer asks for, is how much oil has been filled and how much oil has been drained and the standard reply is "weeeeeeeeeeeell I dont know about the other engineer, but I've tried to fill only as much as I drain each time..." <--- doesnt matter which one of them you ask, you'll get the same answer :D and the "weeeeeeeeeeell" is usually accompanied by either looking at the deck or roof, or some gauges... anywhere but your eyes :)
I've found that staring them unrelenting in the eyes when talking to them works very well to get them to tell the truth :D I know, I'm mean :)
I always look them straight in the eyes and only blink when they look away :) I might come of as some psycho freak to some of them, but atleast I get something I can go by :)
(hate it when others do the eye thing to me tho, makes me feel guilty, even tho I did nothing, thats how I came up with it)
tch... of course I did :DQuote:
Originally Posted by US Iceman
hehehe
Hi guys
Looks like we are having a whale of a time while learning some serious stuff, thanks Tycho for both the fun and the good stuff
simon
Hi, US Iceman
For a room where we can switch the temperatures:)Quote:
Originally Posted by US Iceman
Best regards, Josip :)
Hi, all :)
It seems there are the same problems all around the world...:D minute solutions (enough oil charging:eek: ) and then days to drain the oil to put the plant into operation;)Quote:
Originally Posted by Tycho
Best regards, Josip :)