Redneck High SEER A/C

[Nick1911]

So, as many of you know, I've taken on a/c and refrigeration as a side business.

And, I love it.

I was thinking today: Let's say I take the output of my condensing unit, the liquid line, then feed it into ANOTHER condensing unit.  No compressor in unit number two, just a condensing coil, and a top fan.

In my mind, this would allow for lower head pressures to still get full condensation, thus improving the efficiency of the system.  The lower head pressure should not screw up the evaporator provided a TXV is in place to meter refrigerant and keep the superheat consistent.

Any thoughts before I try to source a scraped condensing unit? 

[Jim147]

I would look for a smaller second unit, maybe a ton less.

I would want to look at fan draw to see if you are making enough up on the seer to pay for running another fan.

Will your breaker handle a hot start on a compressor and both fans on a low voltage 105 degree day?

You need to keep the gas from trying to expand as it goes from one condenser to the other.

Not saying it won't work, just some things that popped into my head.

jim

[Ron]

Depending on how much condenser you add you may need to cycle one/both condenser fans with a pressure control in order to run the unit on milder days.

Some high seer units have "motor masters" or variable speed condenser fan motors. They actually speed up and slow down the condenser fan motor to maintain a certain liquid line temp (liquid sub cooling).  

You need a certain amount of differential between discharge and suction pressures for maximum efficiency.

[280plus]

Too low of a head pressure will cause too low of a suction pressure which will cause freeze-up of the evap. If you try to add freon to boost suction you will most likely flood refrigerant back to the compressor and that ain't good neither. That's what those headmasters and fan pressure switches are about. They keep the head pressure up so your evap doesn't freeze and you don't flood back. I suppose you could run the second fan on a headmaster and achieve some added efficiency on the really hot days. Will it be worth it in the long run? Can't say. You'd have to experiment with sensor placement if you really want to dial it in.

Plus I wonder about taxing the compressor with the added resistance to flow.

[drewtam]

From a thermo point of view, the extra condensor can be used in 2 different ways.
1. Maintain same compressor work input, but cool the working fluid below saturated liquid
2. Reduce compressor workload by decreasing peak pressure & temperature of the superheated fluid

Nick is talking about #2. #2 reduces the peak enthalpy of the fluid and provides less heat available for rejection. This should reduce the overall efficiency of system.

Let me try and explain in plain english...
Hot, high pressure fluid = high energy fluid.
The reason the fluid is hot is because the compressor has done work on the fluid by pressurizing it. The hot fluid is now hotter than ambient, and can dump the heat. The design of the condenser is slave to the design of the compressor. The condenser should already be designed to be the right size in any normal ambient temps (not normal = 130F outside at 10,000ft altitude). If the compressor heats the fluid less, then there is less heat to be rejected to atmosphere. In such a case, the condenser doesn't need to be bigger, but smaller.

The engineering for efficiency goal is the reverse, to heat the fluid to the maximum that materials and cost allows. Then, create a condenser big enough to dump that high temp, high pressure energy to atmosphere (or some colder reservoir). This achieves maximum efficiency.

Another way to really improve efficiency, the condenser should dump its heat to cold underground pipes. Or dump the heat into a swimming pool (2 birds with 1 stone, heat the pool, cool the house). That way the temperature differences is larger not smaller.