Water misters to improve condenser performance?

[HighPotential]

I am considering making my own distilled water misting system ahead of my condenser/compress unit to get more cooling power out of it. I am planning on triggering it by using a two-stage thermostat inside the living space. The condenser/compressor unit is a high quality, 12 SEER, scroll-type Carrier unit. It is only 2.5 tons for 3300/sq-ft which is why I am trying to get more output on hot days. Has anyone had experience with this?

Thanks!

 

[cjw81]

I would be careful with this one. These units are designed for certain engineering criteria. What type of refrigerant gas is being used? This effects the condensing temperature of refrigerant and the sub-cooling (temp of the liquid comming out of the condenser going to the evaporator)This will then effect the superheat (temp of the gas comming out of the evaporator above it's saturation point). The worst you can do is damage the compressor by causing it to deal with liquid slugging (it is designed to handle gas only) by too much sub-cooling. You CAN do it, but only under certain ambient conditions that will be cooling the condenser, but not overcooling it. You will have to add sensors to avoid going out of the design range. Also call your water utility and find out what is in your water. If it has high contaminates, sulfur, ect, you might be seriously reducing the life of the unit by bathing it in water which regrettably really isn't just water anymore. It is also illegal to use water in most communities for condenser cooling unless it is recycled (cooling towers, spray ponds,ect). In your situation, keeping the condenser super clean, going to an r-30 insulation in the attic would help you a lot more. As an example, Adding an r-30 to an r-19 attic can reduce tonnage from 4-1/2 ton to a 2-1/2 ton unit.

 

[HighPotential]

Thanks for the info...I'll get you the refrigerant type tomorrow. As for the water type, I was going to use a small demineralized water tank and pump so as to avoid any contaminants. The misters would be activated by the second stage of a two stage thermostat. If the set temperature gets exceeded by say, 5 degrees F, the second stage of the thermostat would activate the misters and say have a 2 degree setback. That way the unit would only get mist if it was running beyond its capacity. The attic is extremely well ventillated and uses "tech-shield" radiant reflective roof sheathing which alone cuts internal temps by about 30 deg f. Another wuestion I had is is there anything better to wrap sheet metal ducts, plenums and flex ducts with than that standard fiberglass stuff? I'd love to prove that my 2.5 ton can do it. I've oversized all my ducts, put in bigger cooling lines, and using dampers to balance my air. I will use guages to ensure the unit is operating at the peak point. The condenser is located in a shady, breeze-way and has excellent air-flow. Again, I'll get you that regrigerant type tomorrow. Thanks!

 

[NH3Valves]

Sounds like it could work on hot days with some humidity. Essentially adding water mist drops the ambient temperature (dry bulb) seen by the coil to the saturation temperature of the partial pressure of water vapor in the ambient air (wet bulb). If you are in an extremely dry climate this may drop your condensing pressure too low even on hot days. This could lead to some of the problems mentioned by cjw81. If you are like me in Chicago, the design temperature for evaporative condensers is about 20* F lower than dry condensers on hot, humid days. So on a 95* day, the condenser sees an ambient temp. of about 75*. 75* would likely work within the design limits of the unit, and therefore, your idea would work. I would look closely, though, before I decided how to set the second thermostat depending on your climate.

 

[HighPotential]

Wow, this is getting interesting. I live in Los Angeles, which is relatively very dry. We do have some days of high humidity but mostly it is dry heat. I'd say each year only about 3 weeks are high humidity. So would you say that looping in an outside temprature contact (and maybe even humidity) would allow for greater protection of the unit? This is beginning to sound more challenging. I can program up a small logic controller to factor in outside temp, humidity, and if I find some sensors cheap enough, maybe even read the condenser inlet and outlet pressures in real time. I can then program the logic in the controller to active/deactivate the misting. I'm not sure what my setpoints woud be though for the various values.

 

[NH3Valves]

An easier way would be to determine your temperature constraints and limit the water flowrate to keep yourself safe. Check your head pressure on the coolest day you would use the unit. Then wait for a day that is just hot enough for you to want to use the water (90*?) and extremely dry. Play with your water flowrate (Needle valve) and see how it affects the head pressure. If it does not drop below your cool day pressure, you should be OK. This would determine your limit RH and dry bulb conditions. Then just set the thermostat a few degrees higher than this extreme. This sounds like it could be fun. As you drop the compression ratio (high side P/low side P) of your system you should see more efficiency and higher capacity on those hot days!

 

[HighPotential]

Hmmmmm. I want to follow this since this seems easily do-able without gauges and stuff. Just FYI, I do have all of the manufacturers specs so I suppose I could find the range of inlet outlet pressures. Now, to your method. When you say:

"Check your head pressure on the coolest day you would use the unit."

Let's say that it would be 85 degF and dry. So I'll check the head (outlet) pressure. Then:

"Then wait for a day that is just hot enough for you to want to use the water (90*?) and extremely dry. Play with your water flowrate (Needle valve) and see how it affects the head pressure. If it does not drop below your cool day pressure, you should be OK."

So you are saying that adding water mist will drop the head pressure? I think I understand that too much cooling on the condenser will actually turn the return gas to liquid and damage the pump. My trick is, therefore, to cool the gas right to this point but not beyond. So, to continue, I set the amount of misting so that on this day, the pressure doesn't drop below the previous test day (85 deg F). Then, any day hotter than that, I am safe.

I am now beginning to understand the trick (I think). So here is what I propose. I will use the two stage thermostat and put the water activation signal (the higher temp thermostat setpoint) in series with (contingent upon) a compressor head pressure switch such that as long as the head pressure stays above a certain point, the misting system can activate. Once the pressure drops below the allowable point the pressure switch cuts out the misting until the pressure goes up past the internal pressure switch setback.

What did I say?

 

[ChasBean1]

HighPotential:

Let me preface by saying I like the way you think. You've obviously graduated to the point where anything is within your realm from a controls standpoint. My friend at one time had a cat litter box with a motion detector that ran an exhaust fan.

Cooling in the evaporator is by flashing the refrigerant from a liquid state to a gas state. If your condenser does not currently return the refrigerant to the liquid state (which you have not yet mentioned as being a problem with this unit - e.g., no mention of head pressure trips, etc., and I like your unit size with respect to the square footage) I would trust the unit to do what it's designed to do without meddling. Demineralizing water as you mention will probably provide improvement in the corrosion rate over city water, but even pure deionized water will form some carbonic acid (H2CO2), especially when introduced as a mist onto condenser surfaces (by the way, will the condenser fan also be wetted?). The net effect in time will be degradation of your condenser and associated moving parts in a manner that I think would outweigh any benefits that you mention.

I like the discussion above and all posts are insightful and interesting, but I would recommend you instead catch an Angels game (World Series Champs) or even a Ducks game, who I'm adamantly rooting for to win over the Devils. Clean the evaporator coils, change the filters, and maybe even add a small window AC for design day purposes. -CB

 

[NH3Valves]

ChasBean1 is correct in his statements above, as usual. Adding water to a condenser coil always tends to generate problems with scale, acid, etc... That's why evaporative condensers are not as popular as I feel they should be (too high maintenance). CB's thoughts are accurate and practical. Sometimes my reasoning is clouded by an overwhelming need for engineering adventure. You can probably relate. Anyway, the dangers of accessive subcooling in the condenser are dependant on the controls you have (fan cycling, flooding valve, TXV, crankcase pressure regulator). I'm more of a refrigeration guy so without looking too deeply at your system, adding the mist will make the system behave as it is X degrees cooler outside at a given RH. Head pressure is typically a good indicator of what's going on. So I would try the mist out on a hotter, dry day and see what it does, adjust the mist based on pressure, use a two-stage thermostat, and set it conservatively.

 

[HighPotential]

Cool! No pun intended. I'm at least several months away but I'll dig up the thread and update it. Thanks for all the help. I'll try to be as scientific as possible and relay the info. Anyone recommend a good, cheap pressure switch?

 

[stevenw]

You could use the mister to water the roof. This would lower the roof / attic heat load.

 

[Cooky]

The idea of spraying water mist over a condenser coil has been around for a long time, but its use isn't widespread. The process is called adiabatic cooling.

In the technical papers I have written about this process, the improvement in performance is quite impressive. Based on typical UK conditions, a standard Carrier chiller delivered a COP of about 3, when the adiabatic spray system is activated it increases to over 5.

To avoid over condensing of the liquid refrigerant, it may be neccessary to 'pulse' the water flow on & off. Particularly on such a small system.

Regards,

Cooky

 

[HighPotential]

Cooky, now I'm intrigued again. So, if head end pressure is my concern, would you say that my head end pressure switch to activate/deactivate my misting system would be a sufficient safety control?

From a previous post of mine:

"I will use the two stage thermostat and put the water activation signal (the higher temp thermostat setpoint) in series with (contingent upon) a compressor head pressure switch such that as long as the head pressure stays above a certain point, the misting system can activate. Once the pressure drops below the allowable point the pressure switch cuts out the misting until the pressure goes up past the internal pressure switch setback."

 

[Cooky]

If I understand your application correctly, you are using an air cooled condensing unit and adding a water spraying system.

You must have a dedicated HP cut-out switch, so if all else fails the compressor will be switched off.

On the subject of controlling the system head pressure, it may be helpful to realize that when the water spray is activated, the heat rejection from the condenser increases almost immediately by 35% to 40%. Which will cause a rapid reduction in the head pressure (hence the suggestion to pulse the water).

You may need to have more than 2 stages of control if you hope to achieve good control of the head pressure. Maybe you need to experiment to find the best solution, as you are using such a small system.

Regards,

Cooky.