Results 1 to 6 of 6
  1. #1
    pooleshark's Avatar
    pooleshark Guest

    Optimal Hot Gas Defrost?



    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



  2. #2
    Join Date
    Sep 2008
    Location
    Canada
    Posts
    740
    Rep Power
    12

    Re: Optimal Hot Gas Defrost?

    Quote Originally Posted by pooleshark View Post
    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.

  3. #3
    pooleshark's Avatar
    pooleshark Guest

    Re: Optimal Hot Gas Defrost?

    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.

  4. #4
    Join Date
    Sep 2008
    Location
    Canada
    Posts
    740
    Rep Power
    12

    Re: Optimal Hot Gas Defrost?

    Quote Originally Posted by pooleshark View Post
    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.

  5. #5
    Join Date
    Mar 2009
    Location
    U.S.A.
    Posts
    300
    Rep Power
    12

    Re: Optimal Hot Gas Defrost?

    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.

  6. #6
    Join Date
    Sep 2008
    Location
    Canada
    Posts
    740
    Rep Power
    12

    Re: Optimal Hot Gas Defrost?

    Quote Originally Posted by sterl View Post
    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.

Similar Threads

  1. Replies: 12
    Last Post: 27-03-2011, 04:12 AM
  2. maytag fridge activation of new controller
    By jdrefrigeration in forum Domestic
    Replies: 5
    Last Post: 09-03-2010, 12:00 PM
  3. Toshiba RAV SM/SP1400 defrost problem
    By SteinarN in forum Heat Pumps
    Replies: 11
    Last Post: 20-02-2008, 04:25 PM
  4. No Power for Defrost Timer Motor
    By kln in forum Domestic
    Replies: 6
    Last Post: 21-01-2008, 08:22 AM
  5. Hot gas defrost vs Electric defrost
    By herdian in forum Industrial
    Replies: 19
    Last Post: 13-08-2007, 11:25 PM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •