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  1. #1

    Environmental Test Chamber -- Cascade -- Low stage not working

    My employer is the equipment owner. I found this site while troubleshooting my chamber and have learned a lot from reading the threads on here. At this time I do not have any of the specialized tools required to truly troubleshoot refrigeration equipment, so all I can do is guess.

    Some basic facts:
    Tenney BTC with 5 cubic foot work space.
    Air cooled high stage condenser.
    For the intents and purposes of this discussion the work space is empty.
    This chamber has been refurbished.
    From what I can tell, the machine is about 20 years old (the only background I have for guesstimating a machine's age based on its patina is from working on old welders).
    No way to know for sure because the refurb co. stripped the original nameplate that had the s/n.
    During refurbishing, new 1 HP Copeland hermetic compressors were installed
    -New Watlow F4 controller installed
    -High stage refrigerant is R404A (same as original according to TPS who now owns Tenney), 36 OZ charge
    -Low stage refrigerant is R508B (aka Suva 95), static charge 75 psi at 75F (originally was R23)

    There is a large condenser exposed to ambient on the back of the machine. There are two evaporators (one finned assembly with two separate coils) inside the work space -- one is the high stage and one is for the low stage. There is also a dehumidify coil in the work space that is part of the high stage. The intercooler (or evaporator/condensor) is tucked away in an insulated space behind the work space. The manual describes a system used to provide time proportioned cooling power that diverts refrigerant back to the compressor before it goes to the evaporator (I believe you guys call it a hot gas bypass).

    Let me know if I need to describe more.

    The problem: the chamber cools slowly and cannot reach as low of a temperature as it should.

    I have performed several "validation" runs with the work space empty, full cooling power on (I program a -77C set point in the controller), and I manually record the time and the temperature as measured by the controller, to create a characteristic curve of this chamber's cooling capacity. Before doing the validation run, I make sure the chamber has been warm for several hours to make sure the evaporator isn't iced.

    My chamber changes from room temperature to -40 in about 70 minutes. It's supposed to do it in 15 minutes. It goes from room temperature to -20C in about 20 minutes.

    My chamber's lowest temperature is -53.6C. It's supposed to be -73C. It reached that temperature after running for three hours. When left to run for an hour and a half more, it began slowly warming back up (-52C).

    I have already eliminated all the low-hanging fruit -- ambient condenser fan works, adequate clearance from walls, the circulation fan in the work space works, controller is sending the "on" signal to the refrigeration system, pressure safety switches are not opening...let me know if I left anything else out.

    When the refrigeration system was running as described above, the high stage compressor had frost on the housing surrounding the suction line. The low stage compressor was hot (about 170F) and did not even have condensation near the suction line. This makes me think that the low stage is not working properly. I know if could be any number of things:
    -hot gas bypass stuck open
    -oil accumulated in the evaporator
    -restriction in the cap tube
    -leaky valves in the compressor
    -loss of refrigerant charge

    Can anybody comment on this situation? Am I on the right track in assuming that problem lies with the low stage? I would expect the suction line of the low stage to be even colder than that of the high stage.

    Thanks!

  2. #2
    Join Date
    Apr 2012
    Location
    winnipeg
    Posts
    1,330
    you need to check the static charge on the low stage side first
    it was working.... played with it.... now its broke.... whats the going hourly rate for HVAC repair

  3. #3
    Join Date
    Aug 2006
    Location
    st.petersburg,fl
    Posts
    803

    Lightbulb

    Quote Originally Posted by carmon View Post
    you need to check the static charge on the low stage side first
    With a stubby adapter not gauges with hoses so you don't remove any of that expensive refrigerant.
    Isn't sanity just a one-trick pony anyway? I mean, all you get is that one trick, rational thinking, but when you're good and crazy, well, the sky's the limit!

  4. #4
    Thanks, carmon and skpkey9.

    I measured the pressure of the low stage charge and found it to be 64 psi at 65F. The nomenclature plate says it should be 75 psi at 75F. Compensating for the temperature difference, my measurement comes out to be 65.22 psi. So it's about 10 psi low.

    Is that enough to make a difference?

  5. #5
    Join Date
    Aug 2006
    Location
    st.petersburg,fl
    Posts
    803

    Lightbulb

    Quote Originally Posted by quicksparks View Post
    Thanks, carmon and skpkey9.

    I measured the pressure of the low stage charge and found it to be 64 psi at 65F. The nomenclature plate says it should be 75 psi at 75F. Compensating for the temperature difference, my measurement comes out to be 65.22 psi. So it's about 10 psi low.

    Is that enough to make a difference?
    Answer to your question : Yes
    Isn't sanity just a one-trick pony anyway? I mean, all you get is that one trick, rational thinking, but when you're good and crazy, well, the sky's the limit!

  6. #6
    Quote Originally Posted by skpkey9 View Post
    Answer to your question : Yes
    I see what you did there! Ok, to be more clear, do you think the low charge is responsible for all of the loss in cooling power? Or is it only a contributing factor? I realize you might not be able to answer that because you can't get your hands on the machine, but maybe you have a hunch. Thanks!

  7. #7
    Join Date
    Sep 2002
    Posts
    12,571
    I don't understand this part of what you wrote:

    "There are two evaporators (one finned assembly with two separate coils) inside the work space -- one is the high stage and one is for the low stage."

    Why would the high stage have an evap in the work space? This seems counter-productive to me.


    You also wrote: "The intercooler (or evaporator/condensor) is tucked away in an insulated space behind the work space."

    This "intercooler" should be the only evaporator of the high stage - right? So what is the "high stage evaporator" in the conditioned space?

    Are the charge volumes correct?

    What happens if you close off the HGBP line?

    stephen
    ----------




    Quote Originally Posted by quicksparks View Post
    My employer is the equipment owner. I found this site while troubleshooting my chamber and have learned a lot from reading the threads on here. At this time I do not have any of the specialized tools required to truly troubleshoot refrigeration equipment, so all I can do is guess.

    Some basic facts:
    Tenney BTC with 5 cubic foot work space.
    Air cooled high stage condenser.
    For the intents and purposes of this discussion the work space is empty.
    This chamber has been refurbished.
    From what I can tell, the machine is about 20 years old (the only background I have for guesstimating a machine's age based on its patina is from working on old welders).
    No way to know for sure because the refurb co. stripped the original nameplate that had the s/n.
    During refurbishing, new 1 HP Copeland hermetic compressors were installed
    -New Watlow F4 controller installed
    -High stage refrigerant is R404A (same as original according to TPS who now owns Tenney), 36 OZ charge
    -Low stage refrigerant is R508B (aka Suva 95), static charge 75 psi at 75F (originally was R23)

    There is a large condenser exposed to ambient on the back of the machine. There are two evaporators (one finned assembly with two separate coils) inside the work space -- one is the high stage and one is for the low stage. There is also a dehumidify coil in the work space that is part of the high stage. The intercooler (or evaporator/condensor) is tucked away in an insulated space behind the work space. The manual describes a system used to provide time proportioned cooling power that diverts refrigerant back to the compressor before it goes to the evaporator (I believe you guys call it a hot gas bypass).

    Let me know if I need to describe more.

    The problem: the chamber cools slowly and cannot reach as low of a temperature as it should.

    I have performed several "validation" runs with the work space empty, full cooling power on (I program a -77C set point in the controller), and I manually record the time and the temperature as measured by the controller, to create a characteristic curve of this chamber's cooling capacity. Before doing the validation run, I make sure the chamber has been warm for several hours to make sure the evaporator isn't iced.

    My chamber changes from room temperature to -40 in about 70 minutes. It's supposed to do it in 15 minutes. It goes from room temperature to -20C in about 20 minutes.

    My chamber's lowest temperature is -53.6C. It's supposed to be -73C. It reached that temperature after running for three hours. When left to run for an hour and a half more, it began slowly warming back up (-52C).

    I have already eliminated all the low-hanging fruit -- ambient condenser fan works, adequate clearance from walls, the circulation fan in the work space works, controller is sending the "on" signal to the refrigeration system, pressure safety switches are not opening...let me know if I left anything else out.

    When the refrigeration system was running as described above, the high stage compressor had frost on the housing surrounding the suction line. The low stage compressor was hot (about 170F) and did not even have condensation near the suction line. This makes me think that the low stage is not working properly. I know if could be any number of things:
    -hot gas bypass stuck open
    -oil accumulated in the evaporator
    -restriction in the cap tube
    -leaky valves in the compressor
    -loss of refrigerant charge

    Can anybody comment on this situation? Am I on the right track in assuming that problem lies with the low stage? I would expect the suction line of the low stage to be even colder than that of the high stage.

    Thanks!
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  8. #8
    Join Date
    Apr 2010
    Location
    San Diego, CA
    Posts
    218
    Why would the high stage have an evap in the work space? This seems counter-productive to me.
    Some chambers will run only the high stage during humidity testing. This one might be like that.

  9. #9
    Join Date
    Sep 2002
    Posts
    12,571
    Thanks. I am always better with the machines actually in my hands and under my direct scrutiny. Very often I wish that the equipment people ask about was closer so I could run over there and see it.

    PHM
    ------





    Quote Originally Posted by cmclifton View Post
    Some chambers will run only the high stage during humidity testing. This one might be like that.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  10. #10
    Hi Mikey, thanks for your interest. cmclifton is correct about the high stage evaporator.

    About the intercooler: it has an evaporator for the high stage and a condenser for the low stage so the R404A cools the R508B. But I'm just starting to learn about this machine so I could be wrong!

  11. #11
    Join Date
    Sep 2002
    Posts
    12,571
    If I were there, and lacking any specific direction, I would first make a sketch of all the piping and parts in the refrigeration system. From that could be deduced what configurations of operation are possible.

    Then I would configure the system for lowest temp operation and start the high stage. After a few minutes to stabilize I would start the low stage. With the both running I would want to measure all the operating pressures and temperatures and write them down.

    If they seemed 'off' to my refrigeration sensibilities I would then move to correct the things which created the apparently abnormal operation. Low SC/high SSH - add refrigerant, etc.

    With that low side refrigerant the vapor pressure is almost certainly too high for the system components - it's over 3500 lbs. So the internal system/piping volume will have been designed large enough to prevent liquid at static ambient conditions. That kind of a charge is verified by static system pressure - so? It is correct now?

    Can you operate the system and make a list of all the standard refrigeration temperatures and pressures?
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  12. #12
    Quote Originally Posted by Poodle Head Mikey View Post
    If I were there, and lacking any specific direction, I would first make a sketch of all the piping and parts in the refrigeration system. From that could be deduced what configurations of operation are possible.

    Then I would configure the system for lowest temp operation and start the high stage. After a few minutes to stabilize I would start the low stage. With the both running I would want to measure all the operating pressures and temperatures and write them down.

    If they seemed 'off' to my refrigeration sensibilities I would then move to correct the things which created the apparently abnormal operation. Low SC/high SSH - add refrigerant, etc.

    With that low side refrigerant the vapor pressure is almost certainly too high for the system components - it's over 3500 lbs. So the internal system/piping volume will have been designed large enough to prevent liquid at static ambient conditions. That kind of a charge is verified by static system pressure - so? It is correct now?

    Can you operate the system and make a list of all the standard refrigeration temperatures and pressures?
    We work with low pressure air systems, so we have a variety of gauges and fittings around the shop. I was able to make a stubby adapter to measure the static charge of the low stage without losing any refrigerant. I found it to be 10 psi below the spec on the nomenclature plate (65 psi instead of 75). I lack any refrigeration-specific equipment, so I am very hesitant to start measuring pressure at other points.

    I have tried to find a schematic on the internet, but can't get anything specific to this machine. I have tried to map it out while being minimally invasive, but not 100% sure about anything. Nothing is out in the open to be readily observed -- it's all behind panels packed with insulation.

    For example, I thought that the low stage and high stage both had evaporators in the work space, so that depending on mode of operation, the low stage could be disabled. I based that on statements in the manual regarding humidity control (e.g. like what cmclifton said), and a conversation with TPS technical support to a similar effect. Also, the finned evaporator assembly in the work space has two distinct coils, with four lines passing through the chamber wall. That is another reason why I assumed that one was an evaporator for the high stage and one was an evaporator for the low stage.

    After another conversation (since my last post) with a guy at the company that refurbished the chamber, I'm not so sure. He insisted that the finned assembly I've been looking at as two evaporators is really just one evaporator for the low stage only. He said the high stage only has an evaporator for cooling the low stage refrigerant. However, there is a dehumidify coil in the work space that is technically an evaporator for the high stage.

    Mikey, I programmed the controller to 5C and turned the chamber on. This was with the intent of understanding the sequence of events to turn on the refrigeration system and reach a moderate set point. First the fans and the high stage compressor turned on. Then after about 15 seconds the low stage compressor turned on.

    The high stage compressor's steel casing immediately felt cool, and shortly after the low stage compressor started, it felt warm. This is consistent with my observations during a 3-hour run down to the chamber's lowest temperature (-53.6C), when the high stage developed frost on the compressor housing and the low stage stayed hot. There is some cooled refrigerant flow through the suction line of the low stage compressor, because when I compress the insulation by squeezing it with my fingers, I can sense a temperature gradient. It feels like there just isn't enough flow to cool the compressor considerably.

    With these two findings (low stage static charge being 10 psi low, and low stage compressor running hot), is there enough information to say something is probably broken? I don't have any experience troubleshooting refrigeration systems, so I don't know the severity of these indications. i.e. if they are acceptable or 'within tolerance,' or if they are way out of the norm.

    I'm just trying to justify calling in an expert at this point, which I was holding off on doing because the refurbisher says the performance I'm getting from the chamber is typical, but at this point I think it's performing poorly.

    Thanks!

  13. #13
    Join Date
    Sep 2002
    Posts
    12,571
    So the manufacturer's position is that a chamber listed to get down to more than -70 F. in an hour - which will barely get below -50F. in three hours is performing in a standard "typical" manner? If so; it would be difficult for me to imagine an engineering department with a wider standard deviation. <g>

    Always do the easy things first - static pressure is 65 when it is supposed to be 75. So I would start there. Let's see if any of the whiz kids here know if 508B fractionates too much to top off. Either way; in my crystal ball I am starting to see a high pressure refrigerant purchase in your future. <g>

    How big is this thing physically? The smallest ones I typically worked on were about the size of a small van. Although some were the size of buildings. <g>
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

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