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pooleshark
18-11-2010, 03:02 AM
For NH3 hot gas defrost in a holding freezer (0F space target, liquid recirc system), what do you believe is the best method to:

1) Initiate a defrost cycle
2) Regulate defrost cycle (to prevent HG directly entering suction line)
3) Terminating a hot gas defrost

Segei
18-11-2010, 05:40 PM
For NH3 hot gas defrost in a holding freezer (0F space target, liquid recirc system), what do you believe is the best method to:

1) Initiate a defrost cycle
2) Regulate defrost cycle (to prevent HG directly entering suction line)
3) Terminating a hot gas defrost
1. Initiate defrost when you have a frost.
2. Liquid drainer.
3. Terminate defrost when frost is melted. :D

pooleshark
19-11-2010, 03:15 AM
Lol, thanks Sergei, but a common response from most maintenance personnel I deal with at industrial refrigeration sites. To a lot of operators and even consultants, optimizing hot gas defrost is voodo. I was hoping for a meeting of the minds on this one, I've heard of several interesting methods, and everyone seems to have their own opinion of which one is best for each of the three categories I listed above.

Segei
19-11-2010, 06:52 AM
Lol, thanks Sergei, but a common response from most maintenance personnel I deal with at industrial refrigeration sites. To a lot of operators and even consultants, optimizing hot gas defrost is voodo. I was hoping for a meeting of the minds on this one, I've heard of several interesting methods, and everyone seems to have their own opinion of which one is best for each of the three categories I listed above.
Actually it is complicated issue. Look at the thread"Energy saving tips."I've posted some articles.

sterl
30-11-2010, 07:04 PM
RE: Initiating a Defrost:
Lots of work done on that one over a long period of time...Latest I believe is the Intellifrost approach by Logix.

As for return arrangements: Outlet Pressure Regulator and Drainer works well though some evaporators still will have tendency to "steam" and there is some bypass of vapor required unless the unit is designed and can really be installed to free-drain to the float valve. Though its often considered expensive in installation terms, I believe the best arrangement for large localized coils includes a 4th pipe to a liquid accumulator that becomes a full liquid pressure economizer setup.

As to terminate: for a cold storage, terminate when the meltwater stops flowing....We used to look at the first time the probe heater on the trap was satisfied after hot gas injection. The heater was fairly vigrous and still a cheap installation so once water stopped flowing it warmed up pretty quick.

I believe that opportunities to make a more effective defrost and to collect frost before it gets to a coil abound. Even to the point of placing a removable metal grid as more or less a deck on the inside of the doors above the header....Let it frost all day, then remove and let it defrost itself...Or replace if plant functions 24-hours. Any frost it collects, didn't get to the finned coils.

Segei
01-12-2010, 03:00 PM
RE: Initiating a Defrost:
Lots of work done on that one over a long period of time...Latest I believe is the Intellifrost approach by Logix.

As for return arrangements: Outlet Pressure Regulator and Drainer works well though some evaporators still will have tendency to "steam" and there is some bypass of vapor required unless the unit is designed and can really be installed to free-drain to the float valve. Though its often considered expensive in installation terms, I believe the best arrangement for large localized coils includes a 4th pipe to a liquid accumulator that becomes a full liquid pressure economizer setup.

As to terminate: for a cold storage, terminate when the meltwater stops flowing....We used to look at the first time the probe heater on the trap was satisfied after hot gas injection. The heater was fairly vigrous and still a cheap installation so once water stopped flowing it warmed up pretty quick.

I believe that opportunities to make a more effective defrost and to collect frost before it gets to a coil abound. Even to the point of placing a removable metal grid as more or less a deck on the inside of the doors above the header....Let it frost all day, then remove and let it defrost itself...Or replace if plant functions 24-hours. Any frost it collects, didn't get to the finned coils.
Had a look on Intellifrost. It has 2 modes to initiate defrosting.
1. Based on frost thickness.
2. Based on runtime exceeded.
Frost thickness. It is good idea. This should be reason to initiate defrosting. However, end user should determine this thickness (user-define 100% of the frost). This is bad. When defrost should be initiated? 90%, 80%, 70%... capacity of the coil. Surprisingly, this the most complicated question was given to the operator. Assume that we determined that 80% of coil capacity is optimum point to initiate defrost. How should we know that capacity is 80%?
Runtime. During holidays or weekends coil can be defrosted on simple runtime. One graph shows that defrost was done at 10% of the frost thickness. Why to defrost at 10%? Do not defrost if you don't have a frost.
I found that very often people move responsibility of optimization to the operators. This is what customer wants. Did you give him advice about optimum frost thickness or optimum coil capacity to initiate defrost? This approach will not solve the issue.