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  1. #1
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    once more about oversized evaporator



    Hello
    To fight air dehumidification there is often offered as remedy an oversized evaporator. But what about the TEV - should it correspond to the size of the evaporator (so I believe) or to the compressor ?
    Is not the compressor overloaded by moving more gas?



  2. #2
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    Re: once more about oversized evaporator

    Evaporator TD governs the sensible heat ratio of evaporator and dehumidification of air.

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    Re: once more about oversized evaporator

    Whose capacity should match the TEV ? Now I start to think of the compressor and the load. Correct ?

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    Re: once more about oversized evaporator

    Mass flow is driven by the compressor. TEV rated according to mass flow. Put a 10 TR evap on a 5 TR compressor and its a 5-TR evap.....

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    Re: once more about oversized evaporator

    Maybe TEV is at all redundant here : supeheat is no issue with the big evap ? That is, with the 5 evap compressor and 10 tr evap just put in a 5 tr restriction and that's it ? Of course it may be necessary to keep the TD (let's say, 5 K) with an inverter on the evap's fan(s)

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    Re: once more about oversized evaporator

    confused.com

    if you only increase evap size, surely this would be starved with the same size valve and increasing valve size would cause flood back as the compressor could not handle the increase in mass flow and in turn the condenser will not be able to handle the increased duty required

    I thought you had to oversize the whole system to prevent dehum

    So the space is cooled faster than the moisture can be removed from the air

    ???

    R's chillerman
    If the World did not Suck, We would all fall off !

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    Re: once more about oversized evaporator

    in order for humidity to condense we need a colder then ambient coil.

    With a 3 ton system and a 3 ton evap, we can meet this needed Td to condence the humidity.

    But if we put on a 5Ton evap and all other parts of the system remained constent at 3 ton, or Td will now be lower, thusely the evap will be seen as warmer to the humidity reducing its desire to condence, yet we will still effectively remove sensible heat from the air.

    Now lets say we did this in a system but now we do want more humidity removal? We can now increase the Td by lowering fan speed to reduce air flow allowing higher humidity remover.

    If we have a 3ton system, and do not want to change the system but want less humidity removal and ducting allows, we can increase fan speed to achive this goal.

    So to alter the Rh value via evap Td we have wo main common avenues, less common would be hot gas injection but this is geting over complicated for an air con
    Last edited by The MG Pony; 26-08-2013 at 05:44 PM. Reason: added more cenarios
    Now in Redvers Sask.

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    Re: once more about oversized evaporator

    must be having a blonde day all I see with a 5t evap on a 3t system is a starved evap with 3t duty

    if the target is to raise evap temp, why not just fit an epr

    R's chillerman
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    Re: once more about oversized evaporator

    Quote Originally Posted by chillerman2006 View Post

    if the target is to raise evap temp, why not just fit an epr

    R's chillerman
    think the penny just dropped

    epr on same size evap will reduce duty but if fitted to oversize evap you can maintain flow rates/heat transfer

    R's chillerman
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    Re: once more about oversized evaporator

    There is no such such thing as a 5ton evap, an evap can do 5ton at a set of conditions. The same evap can do alot or alot less if you change the conditions.
    The expansion valve is sized to meet the system conditions. Mass flow (duty), pressure difference & temps
    If you are after a small TD, then you may struggle to achieve enough superheat,

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    Re: once more about oversized evaporator

    Something else to beware of is oil return. There might be excessive oil in the coils due to reduced flow. Oversizing a condenser is much less likely to cause problems since the condensing refrigerant makes a good solvent.

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    Re: once more about oversized evaporator

    The honourable MF stated:
    If you are after a small TD, then you may struggle to achieve enough superheat,
    Hi MF - long time, no hear.

    Can you expand on on your logic a little more?
    Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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    Re: once more about oversized evaporator

    NiHaoMike mentioned:
    Something else to beware of is oil return. There might be excessive oil in the coils due to reduced flow. Oversizing a condenser is much less likely to cause problems since the condensing refrigerant makes a good solvent.
    A very interesting point. Good one. I'd imagine the evap circuiting would also need to be set up to minimise oil entrapment.

    Amazing the small things we often miss in a design, or retrofit.
    Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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    Re: once more about oversized evaporator

    quite being pandantic! lol, it was an example and for the most part I am sure most got the reference some times there can indeed be too much info. for the simple air cons they rate the evap at its max designed capacity at what ever that comes out to be, Around here it is 1/1.5/2/2.5/3/3.5 so on.
    Now in Redvers Sask.

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    Re: once more about oversized evaporator

    Quote Originally Posted by desA View Post
    Hi MF - long time, no hear.

    Can you expand on on your logic a little more?
    Hi DesA, ah yes been a busy boy (well not quite a boy any more damm it)

    Most refrig system refrigerant flow is controlled by superheat, so if you want a very small TD or is td (I can never bloody remember), then it is difficult to obtain a high enough superheat for control purposes. (especially when looking for rh well into the 90%+ mark)

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    Re: once more about oversized evaporator

    Quote Originally Posted by The MG Pony View Post
    quite being pandantic! lol, it was an example and for the most part I am sure most got the reference some times there can indeed be too much info. for the simple air cons they rate the evap at its max designed capacity at what ever that comes out to be, Around here it is 1/1.5/2/2.5/3/3.5 so on.
    No offence given.
    The only "tons" I get is "tons of crap from her indoors."
    But on serious not, it would seem nominal ratings vary from area to area and/or application to application.
    So I have no idea what X ton evap is?

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    Re: once more about oversized evaporator

    Thanks MF.

    Thinking through along your train of thought & following Magoo's Rule - where SH is a % of TD, the smaller the TD, the smaller the SH - & so, the more difficult to control.
    Engineering Specialist - Cuprobraze, Nocolok, CD Technology
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