I am going to give you a variation of a trick invented by Poodle Head Mikey that I think might work for you.

Put an EPR valve on the suction line and set it to the refrigerant pressure that corresponds to 33° F. Then set the blower fan to be in low speed during cooling mode. The low fan speed and EPR valve will make the evaporator run just above the point of freezing. The EPR valve will then throttle down the flow of refrigerant, which in turn will cause the compressor to balance itself to better match the load, in effect reducing its capacity when needed. On hot days when there is more load, the EPR will open up and allow full refrigerant flow again.

If you really wanted to get fancy, you might be able to install a 2 stage thermostat and relay, to make the fan run on low speed with first stage cooling, and medium or high for second stage.

Of course, for the EPR valve to work correctly on a heat pump, it would need to be installed on the true suction of the compressor, after the reversing valve.

 

The EPR valve will then throttle down the flow of refrigerant, which in turn will cause the compressor to balance itself to better match the load, in effect reducing its capacity when needed.

This really is not a good solution. An EPR is essentially an adjustable/modulating restriction in the suction line. While it will maintain a consistent refrigerant saturation temperature at the evaporator, it will cause the compressor suction pressure to drop well below where it was designed to operate. To do so would cause the compressor to operate at a higher compression ratio, higher discharge temperatures, and shorten compressor life.

The EPR will not cause the compressor to "balance out" to match the load....compressors do not balance out. It will have a lower capacity at a lower suction pressure, but again....AC compressors are not designed to operate in medium temperature refrigeration pressure ranges.

A better solution would be to add a discharge bypass valve, setting it to prevent the low side pressure from falling below whatever point you want to be the minimum pressure....say a pressure corresponding to 33*. This results in accomplishing the same thing on the evaporator side as the EPR....keeping evaporator temperature at a consistent 33* (until the load rises to a point where it causes the evaporator pressure to rise), while insuring that the compressor operates within its design parameter.

As a heat pump, you'd have to inject the hot gas upstream of the reversing valve, and add a de-superheating expansion valve to prevent the compressor from overheating when bypass occurs.

 

Of course, for the EPR valve to work correctly on a heat pump, it would need to be installed on the true suction of the compressor, after the reversing valve.

You also have to cut-in a bypass solenoid synchronized to open in heat mode or it will severely limit or make heat pump function completely useless, because you will limit it the outdoor evaporator to 33F.

 

Charge with an r22 alternative such as mo99, r421a ect and you'll lose ~5-15% capacity or possibly more depending on which one you chose. And/or Slow the blower down or block off return air to get your sst down to just above freezing.

 

After I posted it did occur I had forgotten the flaw of icing up with my simple 'starve the coil method.

 

Rather than reducing cooling capacity, it's relatively easy to simply add reheat with either electric strips or a hot water coil. Short of that, look into adding hot gas bypass.

 

Rather than reducing cooling capacity, it's relatively easy to simply add reheat with either electric strips or a hot water coil. Short of that, look into adding hot gas bypass.

This is a very costly approach. I've seen a legit setup in scientific equipment which must maintain a very constant temperature over a very wide adjustment range without using a huge amounts of water/brine to buffer it. The refrigeration system ran whenever it was powered on and it maintained a constant temperature by rapidly cycling the heater. If process load increased, heater backed off. If process load was almost zero, heater cycling maintained a tie with the refrigeration system. Variable refrigerant was not a technology available for such purpose when that thing was made.

If the person wanting the cheap patch up done and the person paying electricity are different, this is unethical.

Basically I think at least in all but the hottest summer conditions we’d be better off with tempered S/A and longer cool on’ times.

From what the thread starter said, 5 ton of cooling is overkill for majority of the season.

You pay more to get less. When you combine the energy used by heat strips, it'll use about three times the power compared to a 13 SEER system. Depending on climate and utility, you spend enough on wasted power to the extent of what it costs to just by 3.5 ton system.

Assume existing system is 13 SEER, which means COP is about 3.42.
At full load, this tells me the system uses 5.14kW with a 5 ton load.

Say you want to cut capacity to 3.5 tons by injecting 1.5 tons of heat (5.3 kW).

5.14kW AC +5.3kW heater =10.6kW electricity for 12kW cooling, or you get COP of 1.13 or a whopping 4.43 SEER unit.

 

Maybe convert it to use R134a? Still not a cheap option but probably cheaper than replacing the compressor.

 

Thank you all for the ideas. Wouldn't want to do a permanent reduction, and I'd be way more inclined to speeding the compressor down if the money was there.
(beins' brought up' energy reduction round here :grin2:

 

http://www.youtube.com/watch?v=KmyM9kpISIw&feature=c4-overview&list=UUpFDfep36yxunfznRfv-W8A
A R22 to R134a conversion has been done before with good results. It's just a bit labor costly to change the mineral oil to POE, especially if there are many points where the oil tends to collect. A mixture designed to replace R12 would probably work, but such mixes are more expensive than R134a.

Another idea is to add a water coil, pump, and tank. Mount the water coil after the evaporator. The idea is to have the thermal inertia of the water moderate the temperature swings. Could also add a heat exchanger (could be as simple as some copper tubing sitting in the tank, connected between the TXV and existing evaporator) to turn the system into a hybrid air cooler/water chiller. Keep the fan and pump running at all times so that the system has some cooling during the off cycle.

 

I thought about how nice it would be to retro fit a vari-speed system to the compressor. Previously I presumed having to run room inputs, controllers etc. After seeing your vid, and seeing it control off suction temp, I'm wondering if that wouldn't make for a reasonable cost option.
A second input -a solar sensor would be a bonus and could easily be run externally up to the glass. Add the right ratio and we'd have a decent shot at auto-compensation to the warmest end of the conditions. Not a real room average' or what have you, but probably close enough.
Would there be any reason not to stay with r22?
By the way our evaps are above the condensers. That's would be a plus for oil return and extending the safe low speed limit would it not?

 

Would there be any reason not to stay with r22?

Using R134a in a R22 compressor reduces the system capacity if all other factors are the same. (It's basically the Davuluri Treatment but for a R22 system.) You wind up with an "overbuilt" compressor and oversized condenser and evaporator, which means higher efficiency.

Another way to reduce capacity (albeit with some loss in efficiency) is to reduce the airflow across the coil, but you can only go so far before the coil freezes. You'll also need to watch your discharge temps due to a higher compression ratio.

There's also the option of plumbing in a subcooler coil. Efficiency goes down but much less so than adding heat strips or a hot gas coil.

 

Actually the point was to have the reduction most of the time when it's not all needed. Thus the previous post..

 

Then a VFD would be your best bet.

 

How about an EPR combined with liquid injection?

 

Or to quote Dr. Ehsani (a power electronics professor at Texas A&M), "That's like going pedal to the metal on the gas and applying the brakes to control speed."

Assuming changing refrigerant or adding a VFD (or replacing the compressor with a smaller one) is out of the question, a subcooler coil is the most efficient solution. As it offsets some of the cooling, it lowers the discharge pressure to partly offset the efficiency penalty. Next up is slowing down the evaporator fan, followed by a hot gas coil. The heat strips are dead last in efficiency and should only be used as the last step in an application (such as a calibration lab) where temperature control is more important than efficiency.

And also don't overlook just adjusting the dampers to use more outdoor air. Efficiency penalty would be somewhere between the hot gas coil and the heat strips, but you also get fresher air inside.

 

Way to complicate a simple solution.

What happened to installing a new or used 3-ton system, then reappropriating the existing 5 ton system to a properly matched charitable project for tax deduction?

The current owner of 5 ton can just use a plug-in portable unit to boost it for those several days a year extreme cooling needs.

System owner wins, the charity project wins, and depending on how its arranged, system owner gets recognition for donating the system, and contractor gets recognition for donating service.

 

If you re-read what the OP asked, he wanted cheap. He ruled out VS and unloading due to cost. He then suggested tempered supply air...which to me sounds a lot like reheat.

I was wondering about down a dirty (alright not dirty, 'cheap) ways to reduce the cooling cap of a 5ton split heat pump.
Vari-speed (not really cheap
An unloading compressor ( ditto

Could one pull some r22 out enough to knock off let's say 20-30% of the cooling effect w/o causing other troubles?

I have a situation where high solar load makes for terrible comfort issues when the system cycles where the temp swing up on the off cycle equals immediate discomfort.
Basically I think at least in all but the hottest summer conditions we’d be better off with tempered S/A and longer cool on’ times. (A hundred sq ft of south exposure glass per room at 110 degrees, 65 out side ...

 

I think.. (yeah pretty sure ) what the OP wanted to have less capacaity- like in the title? :grin2:
So yeah 'temped' as in not as cold true, but wrong context.

And as for vari speed, I always saw that as attractive, now (since post 11), really hoping to revist that

 

How about a smaller compressor? Oversized coils seem to be the way to go. Or am I missing some vital logistic?

 

Now that ive pondered it for a bit could we actually increase efficiency with a smaller compressor? Thats about what the manufactures have done, decreased the ratio of compression while maintaining volume of heat absorption. Again, please tell me if im way off. Of coarse air flow needs reduced indoors. Out door fan should be fine?