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  1. #1
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    Jul 2009
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    Multi-Stage Dual Compressor vs 2 speed compressor.

    Looking at the Trane high end units Trane XL16i Model 4TWX6024B and Trane XL20i Model 4TWZ0024A.

    Are the first stages of compressor operation the same BTUs but different energy consumptions. I have read that the two speed compressors operate at about 70% in first stage, while the dual compressor models operate at about 50% in first stage. Is this an energy usage statement or a BTU output statement? Is the BTU output close and the difference is an efficiency issue with varying the output of a single compressor?

    Asked another way: Is a dual compressor model more efficient in its first stage compared to a 2 stage single compressor due to the motor being more efficiently matched to its load?

    Does anybody have data on the BTU/Kw per stage for these units available?

    And finially, I have only been able to find dual compressor models available from American Air/Trane. Is there another manufacturer with dual compressor models?

  2. #2
    Join Date
    Dec 2007
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    Cedar Grove, Wi-Sheboygan
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    Might want to read this tread, it might answer some of your questions:
    http://hvac-talk.com/vbb/showthread.php?t=338312

  3. #3
    Join Date
    Jul 2009
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    Dallas, TX
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    Thanks for the suggestion, but nope that did not help. I actually authored that other thread, what I am asking here is something different.

  4. #4
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    Jul 2009
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    I have seen talk about one of the Carrier/Bryant models that have a 50/100 capacity split. So I guess I would like to modifiy my original thread a bit by saying, I am looking for a 50/100 split more than a dual compressor model.

    And on that note, do the dual compressor models stop compressing for a bit while they switch compressors? Do the single compressors need to stop before staging up?

  5. #5
    Join Date
    Nov 2004
    Location
    Austin, TX
    Posts
    873
    The two compressor systems, the first compressor is rated at a lower BTU.
    The blower will operate at a slower speed with the first stage compressor.

    The two speed compressor (if it is a copeland 2 speed scroll compressor) has a solenoid to unload the compressor so it moves less refrigerant. It uses less electricity in the unloaded state. The blower also operates at a lower speed on 1st stage.

    The two compressor system is electrically and mechanically more complex than the two stage copeland scroll.

    In my opinion the two speed bristol reciprocating compressor used on many of the 50/100 Carrier/Bryant systems is more vulnerable to mechanical failure than the two speed copeland scroll compressor used on many 75/100 systems.
    “I am for doing good to the poor, but...I think the best way of doing good to the poor, is not making them easy in poverty, but leading or driving them out of it. I observed...that the more public provisions were made for the poor, the less they provided for themselves, and of course became poorer. And, on the contrary, the less was done for them, the more they did for themselves, and became richer.”
    ― Benjamin Franklin

  6. #6
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    Jul 2009
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    Dallas, TX
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    Allan38,

    I understand that the two speed bristol reciprocating compressor used on many of the 50/100 Carrier/Bryant systems is more vulnerable to mechanical failure than the two speed copeland scroll compressor used on many 75/100 systems, but I am zoning the heck out of this system. One of my zones is quite small, and I really want that 50/100 capacity split.

    It is looking to me like if I want that 50/100 split I either need the Carrier mid line 2 stage system or the Trane high end two compressor system. I will accept mechanical complexity in exchange for that 50/100 split in my application.

    Anyone know of another option I should be looking into?

  7. #7
    Join Date
    Sep 2006
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    1,253
    Quote Originally Posted by Illusion View Post
    Allan38,

    I understand that the two speed bristol reciprocating compressor used on many of the 50/100 Carrier/Bryant systems is more vulnerable to mechanical failure than the two speed copeland scroll compressor used on many 75/100 systems, but I am zoning the heck out of this system. One of my zones is quite small, and I really want that 50/100 capacity split.

    It is looking to me like if I want that 50/100 split I either need the Carrier mid line 2 stage system or the Trane high end two compressor system. I will accept mechanical complexity in exchange for that 50/100 split in my application.

    Anyone know of another option I should be looking into?
    Might want to consider inverter driven compressors where the compressor is completely variable in output from about 25% of nominal capacity to over 100% (how much depends on manufacturer). This allows for the system to vary the compressor output to match the actual gain/loss of the structure and even provide cooling/heating when the load is very light. That is especially good for dehumidification in warm but not hot weather.

  8. #8
    Join Date
    Jul 2009
    Location
    NE Oklahoma
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    Ditto with mchild. Many brands use inverter technology...Daikin is a wonderful piece of equipment. Low amp, "soft starts" and variable capacities based on loads. Rheem prestige also makes dual compressor condensing unit.

  9. #9
    Join Date
    Jul 2009
    Location
    Dallas, TX
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    Thanks for the suggestions. I unfortunatly have to disqualify the very cool inverter based units as they all require their special thermostats to control.

    Using my existing thermostats is a requirement of this upgrade as I have hundreds of programs written that adjust the temperature via RF to said thermostats so I will have to stick with 2 stage units that allow conventional non-communicating thermostat wiring. I will be using the HZ432 Zone controller from Honeywell so the unit needs to be looking for the classic O/B C G R Y1 Y2 W (or whatever for aux/emergency) and I would also like BK as that zone controller will activate that feature when only one zone is calling which is pretty cool.

    I looked at the Rheem info. The info avail to consumers seems to indicate a single 2 speed scroll, and the performance tables show the usual 80% BTU rating in first stage, so maybe I was looking at the wrong units or they have stuff that is not viewable by the general public.

  10. #10
    Join Date
    Mar 2002
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    Just following the convo Illusion, I notice that allan mentioned the complexity of the Trane two compressor system to which you replied with a comment about the two-speed Bristol compressor system. I wouldn’t call a two-speed compressor particularly complex.

    As for reliability, there is the assumption that a scroll is inherently more reliable than a recip. It probably is, but then recips have been chugging along reliably from long before I was born. I’ve got a few two-speed Lennox systems from the early eighties on an account of mine. They’re still running like champs.

    I’m not the multi-stage expert. However, I believe both systems will have a short delay between stages.

    Early this year I went on a hunt for 50/100 staging. I live in a dry climate, so the 70/100 staging is in my mind a pretty horrible idea – even if I just have two zones. Just as you found, Trane and Carrier were the only 50/100 options I could find.

  11. #11
    Join Date
    Apr 2004
    Posts
    7,680
    If you have a dual compressor system where the compressors are equal, there is a slight load carried by the 1st stage that gives you somewhere around 55% capacity. The energy usage is also higher than half since you are running the blower and the Outdoor fan with 1st stage and only slightly increasing them in wattage when the second compressor kicks in.

  12. #12
    Join Date
    Jul 2009
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    Dallas, TX
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    Quote Originally Posted by Irascible View Post
    Just following the convo Illusion, I notice that allan mentioned the complexity of the Trane two compressor system to which you replied with a comment about the two-speed Bristol compressor system. I wouldn’t call a two-speed compressor particularly complex.
    Irascible,

    Thank you for taking the time to reply to my post. It is a particular honor for me to have you comment on my question as I am a huge fan of yours! I have read nearly everything on your website, even the stuff that has major formatting problems. I have been so interested in what you had to say that I copied text off your website with the formatting issues and pasted it into a text editor to be able to read it. I greatly respect your way of thinking and running your company. I am a sole proprietor as well and I am also a one man band in my chosen field. I also strive to be highly technically competent and not just do what works or makes me the most $$, but what is best for the customer. I am sorry that you are not in the Dallas, TX area or I would already have my AC guy for this upgrade.

    Now as for mechanical complexity. It is my understanding that the Carrier 50/100 split system uses a compressor that reverses direction in order to stage. It has two pistons, one of which is engaged in forward rotation and both pistons are engaged in reverse rotation. I do no know how this is accomplished, but certainly this is likely more mechanical complex than a 2 speed scroll that simply unloads the compressor via a solenoid.

    I am not however excluding this unit from my selection process based on this. In fact, I think this unit is the leading contender for my application. I have a piston based compressor right now that I have been very happy with for 15 years and I am not afraid to put a two capacity version in service here. But I do think it qualifies as an increase in mechanical complexity.

  13. #13
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    Mar 2002
    Location
    Concord, CA
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    Thanks for the good word Illusion. I have to admit that I didn't know of the negative feelings towards Bristol until I did a search prompted by your post. My own experience with the TS product has been very good. As I mentioned, they're very old. They're located at a church/school and have been used almost daily since they were installed. plain spoken and jrbenny agree in their own way.

    I'm was going to PM you about the formatting problem. It seems you can't receive PM's.

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