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superheat
21-10-2002, 06:01 PM
I see alot of systems with 1/2 larger evap than CU. Many times they leave the original orifice in the evap. I am quessing it is not going to make much difference. If you charge that system perfectly in the spring when it is 75 degrees outside, how much superheat would you have when it is 95 outside. I would think that your superheat would dissappear when it gets hot out.

frank
21-10-2002, 10:10 PM
One would assume that the more heat outside would also mean more heat inside during the summer months. More heat absorbed into the evaporator would result in higher suction pressures.

If the condensing unit (and one would assume the compressor) is undersized in relation to the evaporator then the evaporator would be starved of liquid refrigerant. This would result in higher superheats as the evaporator would not be full of liquid.

frank

superheat
22-10-2002, 01:40 PM
You charge this system in a situation where the superheat chart calls for say 21 degrees of superheat. After a while the same system is in a situation that the superheat chart calls for 5 degrees of superheat. What will the system superheat be?

Andy
23-10-2002, 12:31 PM
Hi, Superheat:)
I think we are supposed to size the oriface to match the cond unit duty at design conditions, it is therefore likly that the superheat will be above the normal during pulldown if the unit is small compared with the evap, this will lessen when we reach the design conditions.
I am going to put my foot in it here and say that there is no such thing as an oversized evaportor, you will be just designing with a smaller TD in mind. (ofcourse you can over do it with say a 1k design TD)
Larger evaps and condensers will lead to a more effecient system and in the case of the evap a lower latent heat removal.
Regards. Andy.

Gary
24-10-2002, 05:45 AM
Larger evaps and condensers will lead to a more effecient system and in the case of the evap a lower latent heat removal.


Actually, higher latent heat removal at same SST or lower latent heat removal at same fan speed. Or anywhere in between.

reggie
24-10-2002, 09:00 AM
Andy as regards to your quote of larger evaporators lead to increase of latent cooling you do have a point. I worked with a guy who claimed that the design on a number of Japanese heat pump splits would have larger evaps than cooling only counterparts.
Thus they ran at lower evaporating temperatures increasing the total duty but lowering the sensible heat ratio and relied on the coil thermistor for compressor shut down during long running cycles.

superheat
24-10-2002, 01:44 PM
I have always wondered about "mismatch" heat pumps with larger indoor coils. I do not do much heat pump work. I was under the impression that not matching coils exactly messes with the balance of ***** charge between the 2 seasons. ie. system will be overcharged one season and undercharged in the other as measured by compressor superheat.
I am very curious about these Japanese systems.

Andy
25-10-2002, 05:09 PM
Hi, Gary,Reggie,Superheat.
:) I was thinking that if you used any given condensing unit and fitted it with a larger evap, say one working on a 6 TD instead of 8 TD the amount a latent cooling would decrease.
Also when sizing splits for computer rooms you would select an evap one size bigger than that which would be normaly be used.
This is an observation made to me by a friend who works for a wholesaler suplying Mitsi units. This selection is purly to protect the computers from static which is associated with dry air.
Regards. Andy.

superheat
25-10-2002, 05:55 PM
The other variable in the equation is airflow. Larger evap will drop the TD and raise the evap temp. If you decrease the airflow, you can increase the TD.
Imagine a larger evap with less airflow that would give the same evap temp. It would dehumidify more than a small evap with the same evap temp. Clear as mud?

Gary
25-10-2002, 08:54 PM
Exactly so. Given the same coil temperature, a bigger coil means having more surface area upon which to condense moisture.