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  1. #40
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    Sep 2018
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    Quote Originally Posted by jayguy View Post
    i seriously question your advice.
    I don't know what I was thinking.

    Tell me, what would you do with an oversized unit?

    Get more air to it, slow it down, don't use 100% capacity. All seemed reasonable to me. Now tell me, what would you do?

    And if you could tell by his first post, changing it isn't an option. Now tell me.

  2. #41
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    Nov 2006
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    More hot air, not just more air. That would be outside air.

    Only run one stage. That makes it a smaller unit. I am fine with blocking the top half of the evap coil with panning.

    In this case, you can't fix stupid, but you can change how the stupidity affects the building.
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  3. #42
    Join Date
    Aug 2006
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    Mixing oil and fire with a big spoon.
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    Quote Originally Posted by Goldwaves View Post
    Increasing airflow with what already seems like a low charge will not help...
    Low airflow is one of the most common reasons for low suction pressure (like a dirty air filter). Increasing airflow will not help with an oversized unit but decreasing the airflow will make the suction superheat worse and add other issues.


    Quote Originally Posted by Goldwaves View Post
    ...If you do anything with the VFD it would be to decrease airflow which I know seems counter...
    See above. Decreasing airflow sounds counter intuitive....because it is.


    [QUOTE=Goldwaves;25776134]...If you do anything with the VFD it would be to decrease airflow which I know seems counter intuitive because you already have a low suction temperature, but adding refrigerant or more fresh air can fix that part.

    Quote Originally Posted by Goldwaves View Post
    ...If you do anything with the VFD it would be to decrease airflow which I know seems counter intuitive because you already have a low suction temperature, but adding refrigerant or more fresh air can fix that part....
    Adding more refrigerant to a TXV system will not really do anything...good or bad. If anything, it will likely make it worse but not but a lot. If it is an orifice system, adding refrigerant will make the suction line superheat lower and possibly damage the compressor. Add your lower airflow approach and he will be replacing lots of compressors under warranty...for a while.


    Quote Originally Posted by Goldwaves View Post
    ...Adding in some fresh air, slowing down the blower(for SP only), and adding refrigerant can more than likely fix all of your issues...
    So, doing this will fix all oversized units everywhere? Even a unit that is 4 times oversized? That seems a bit oversimplified doesn’t it?


    As as for “what would I do?” I would redo the load calculations and get the correct unit or make correct changes (perhaps it is a ductwork issue as mentioned previously). An improperly sized unit will not be fixed by an improperly sized unit. If it is a little bit oversized, that is ok and corrected by cycling. But grossly oversized just can’t be fixed this way. Just because he says that a new unit can’t happen doesn’t mean that a new unit is off the table. It means he doesn’t want to pay for it. We got our ass handed to us by not paying close enough attention to a chiller overhaul with a new customer. We lost $60k on it but kept the customer. It’s a matter of priorities. Money or reputation. It’s a choice.
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  4. #43
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    Jun 2014
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    OP, you gonna come back and fill us in on your results/findings?
    “I haven’t failed. I’ve just found 10,000 ways that won’t work.” - Thomas Edison

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  5. #44
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    Sep 2018
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    Florida
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    I guess I should have went in order of operation and explained every piece bit by bit...

    It's a 2 stage system. I would eliminate one stage, making it no longer a 15 ton unit.
    Next is to check the charge while operating one circuit. I recommended adding refrigerant because of low sub cool, not superheat.
    Next I'm increasing the air to the unit by using the power exhaust and opening the intake damper. This should increase my superheat.
    The reason I'm also slowing it down is to monitor the system under conditions I'm more familiar with, I'll ramp it up if necessary.
    I like the idea of adding a headmaster or EPR, good suggestions.

    It's not this guy fault the unit wasn't sized properly, and honestly the owners aren't going to just want a new unit.

    But you see, my way would be shortest...

    And would also have the stones to tell the owner exactly what I did and why.

  6. #45
    Join Date
    Feb 2017
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    Thread Starter
    Sorry been out of town for a few days. I took Spitz's advice. I blocked the airflow over the upper evap. Maxed out the vfd to 60 hz. I then gave it some more oa from the economizer. Started the powered exhaust to 15hz and took out some gas to get my superheat up to 10. I still have low suction pressure but above freezing 35 degrees. I am off quite a bit from my charging chart on the unit. I had a 28 degree split across my evap. Question is will it harm the compressor with such a low suction pressure. Btw it doesn't have a txv.

    I really appreciate everyone's advice. I just dont think that adding refrigerant and slowing the blower will help. I work for an university full time. I just started doing hvac last year. My main job is electrical but have been doing alot of control work on campus, Automated logic. I recently took on all utilities. I've been self learning myself so be patient with me. I got another question for a side job I was on this evening. But that will be another post. Condensor ecm motor acting weird.

  7. #46
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    Jul 2018
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    Wisconsin
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    Keep in mind you can run up your VFD over 60hz.. As other have suggested I would also throw in a head pressure control. ICM makes some nice stuff.

    Sent from my SM-G960U using Tapatalk

  8. #47
    Join Date
    May 2006
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    Quote Originally Posted by gravity View Post
    Is your fan spinning the right way?

    Sounds like you need some HGBP or a RAWAL valve.

    I would also install head pressure control / motor master and set for 400psi. This will keep the head up, discharge superheat up, and keep the suction up to a certain point.

    Could even try a EPR.
    2nd for HGBP. A Rawal valve will take that circuit down to about 10-15%of its nominal capacity.

  9. #48
    Join Date
    Nov 2001
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    Just a couple of thoughts off the top of my head -

    If you have enough cooling now, you may want to consider a Rawal valve as a temporary measure. They put a false load on the compressor by directing hot gas through the evap coil. They are designed for allowing systems to turn down without having a variable-capacity compressor. It would be a waste of energy, but it would get you out of the freezing range.

    I have not used them myself, but have heard a lot of good things about them.

    You may also want to consider putting a smaller compressor in your unit.

    Either way, you would need a very knowledgeable tech to support you. Neither of these are ideal solutions, though the smaller compressor would be a better choice if that is technically feasible.

    https://www.rawal.com/

  10. #49
    Join Date
    Dec 2014
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    RAYWAL is not designed to reduce capacity. It is designed to add some load to maintain a 35-degree evaporator during those times when that is necessary.
    It actually mixes hot gas with TXV metered efrigerant, to create an adiabatic process .
    This happens close to the compressor and one of the advantages is that you don't need to run a hot gas line to the evaporator coil. It can save a lot of copper on...say, a 100 foot split.

    They are not made to save somebody's butt in the case of a vast oversizing.


    Sent from my SM-G950U using Tapatalk

  11. #50
    Join Date
    Feb 2006
    Location
    Phoenix,AZ
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    This guy needs to see whats up with his airflow. He may even have a fire damper shut. If he doesn't he can add on.

  12. #51
    Join Date
    Nov 2001
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    Quote Originally Posted by icy78 View Post
    RAYWAL is not designed to reduce capacity. It is designed to add some load to maintain a 35-degree evaporator during those times when that is necessary.
    It actually mixes hot gas with TXV metered efrigerant, to create an adiabatic process .
    This happens close to the compressor and one of the advantages is that you don't need to run a hot gas line to the evaporator coil. It can save a lot of copper on...say, a 100 foot split.

    They are not made to save somebody's butt in the case of a vast oversizing.
    I agree that is not what they are intended to do, but putting the false load on the compressor does just that. The compressor works just as hard, but you get less cold out of the coil. That's how they allow units to run at very low loads. Again, it is just a thought as a stopgap, as the OP would be paying to run 15 tons of compressor to get 5 tons of cooling.

  13. #52
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    Jan 2004
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    PA
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    Quote Originally Posted by coolcoil View Post
    I agree that is not what they are intended to do, but putting the false load on the compressor does just that. The compressor works just as hard, but you get less cold out of the coil. That's how they allow units to run at very low loads. Again, it is just a thought as a stopgap, as the OP would be paying to run 15 tons of compressor to get 5 tons of cooling.
    The RTU the OP is working on. Has 2-7.5 ton circuits/compressors.

    When the Rawal valve is maintaining the evap pressure/saturation. The compressor is not working as hard as it would if it was under an actual load. While it raises/keeps the suction pressure up, the head pressure drops, reducing compression ratio. So the compressor is doing less work.

    The system the OP is working on, could have a fire damper or 2 closed or partially closed. A Rawal valve would cover this up. And the customer would end up with a system that doesn't cool the area like it should.

    Remember, the RTU is freezing up, with only a single 7.5 ton circuit running. Something is up with the duct system. Or, they didn't get the correct curb adapter and supply air is being blown back into the return opening under the RTU.

    Rawal valves have their place/uses. But they aren't a cure all.
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