once more about oversized evaporator

[Yuri B.]

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?

[Magoo]

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

[Yuri B.]

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

[sterl]

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.....

[Yuri B.]

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)

[chillerman2006]

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

[The MG Pony]

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

[chillerman2006]

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

[chillerman2006]

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

[mad fridgie]

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,

[NiHaoMike]

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.

[desA]

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?

[desA]

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.

[The MG Pony]

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.

[mad fridgie]

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)

[mad fridgie]

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?

[desA]

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.