Condensing Temperature Delta T vs SEER

[congo40]

I'm trying to nail down what the actual condensing temperature delta T is for modern residential air-cooled condensing units. I'm not an HVAC expert and it is my understanding that a typical SEER 13 air-cooled condensing unit maintains a 20 deg. F delta T above whatever the outdoor dry bulb temperature happens to be. Is this true? If so, does a SEER 15, 16 or 19 unit have progressively smaller delta T's, or do they derive their efficiency improvement from other factors? In other words, does a SEER 19 system also have a 20 deg. F delta T, but it's efficiency gain comes from perhaps a larger condenser or improved compressor performance?
Also, any guidance on how to obtain performance graphs(e.g., efficiency vs outdoor drybulb) for various SEER rated units?

 

[gepman]

See the recent thread

http://www.eng-tips.com/viewthread.cfm?qid=204535&page=2

for a somewhat related discussion.

A 20 - 30 deg. TD for the condensing coil (at design conditions) is typical for these units but there is no hard number. Perhaps there are some engineers from the package unit industry that can answer this better. The actual TD will vary depending on the suction and discharge conditions because the compressor capacity varies with these conditions. Condensing coils are usually rated in BTU/H per deg. TD.

The manufacturers have their options on how to improve the EER or the SEER rating. One good method as you stated is to reduce the TD on the condensing coil. The same can be done on the evaporator coil which increases efficiency also. They call this "adding copper" to increase efficiency. The other methods of increasing efficiency are to put more efficient outdoor and indoor fans. They have been very inefficient (sometimes less than 50%) but manufacturers are starting to put in better fans, even variable speed motors. The last option is to put in better compressors however the good compressors are relatively efficient and it is becoming more difficult to improve compressor efficiency substantially.

On larger systems you can modify the control scheme to make it more efficient however small package units are usually constant volume on/off (non-modulating).

You can get some information on package units from the manufacturer. I have attached one for a typical Carrier 2 ton unit.

 

[EmeraldCoastHVACR]

Condensing temps for lower seer units based on ARI standards:

7 to 10.5 seer -- 25 to 30 deg
10.5 to 11.9 seer -- 20 to 25 deg
12 & up seer -- 15 to 20 deg.

These are the numbers I use in the field in engineering & contracting for diagnosing hvacr problems.

Very difficult to get below 15 deg on any seer unit, this is why we have water-based condensing units.