View Full Version : 100% outside air units
08-04-2004, 09:33 PM
has anyone here had any dealings with witt outside air equipment.i have a problem that has me chasing my tail.this particular installation is on a school feildhouse(weightroom).this unit,a 12 ton witt is being used as primary air conditioning for this building.i have other schools where they are being used as fresh air units but this is the only one i have that is using 100% outside air as PRIMARY cooling.the problen is the compressor(copeland semi) keeps failing on a oil pressure switch.i know(or think)this is being caused by liquid flooding because there is foam in the comp. sight glass.the thing is that all checks show the unit to be operating correct.75-80 suc press.25* superheat and sweat back all the way to the comp,oil press of 30-35#.i can never manage to be at the unit when it fails but when i am there it seems to be doing fine.i am from florida and this time of year we have 95-100 degree days with 75%+hum.i just dont think the piece of equipment is capable of dehumidification and cooling the building with such extreme temps.we are just about to the point of changing the unit(i am not a fan of witt units anyway)but i am going to meet with the engineer of the building before we do.does anyone out there have equip. preforming in such conditions and if so what kind of unit seems to be the most reliable in these extreme conditions. HELP!!!!! THANK YOU
08-04-2004, 10:25 PM
i,ve got some O.R. rooms at a hospital that use 100% O.A.
These are chilled water setups but a cold coil is a cold coil.
Without a load calc for the space and specs on the existing equipment, I can only guess as to what your next
move would be.
As far a changing out the unit, what will you base your reason on?
Lets say this unit runs all day @ 32-30 coil temp.
NIght time arrives and the frost is building.
100% O.A. suction superheat falling, liquid getting back,
oil pressure failure.
You show up, 100% O.a. Melted any frost, and put a big ol load on the space. Start up, all looks normal.
08-04-2004, 11:27 PM
need subcooling and superheat for correct answere but well guess. is it fully loaded, is 35 lbs what this compressor is supposed to pumping. is this net oil pressure. are you sure oil pressure should not be 50 lbs.
flooding,unloaded,low load or airflow across evap, oil screen clogged,broken valve,low head pressure etc..,
you should be able to fiqure out qhats wrong by checking the system.
08-05-2004, 05:45 PM
My thoughts on this:
35 psi of NET oil pressure is ok: the Copeland Oil Pressure Safety Control will shut down in the 9 to 11 psi range [maybe a little higher]. I doubt that you have flood back with the amount of superheat you indicate. IMO- a system this size should be on a pumpdown cycle. Also, ensure that the crankcase heater is working. Both of these will help gaurd against flooded starts as will a suction accumulator. I'm wondering if the electrical circuit has a current sensing relay that monitors current flow to the compressor? The relay contacts are wired to one side of the safety heater circuit in the oil failure control. If the compressor shuts off on it's internal protector but the compressor contactor is still pulled in, you will get a nuisance oil failure trip. So, for example, if the compressor is overloaded due to handling the outdoor air load, this might be what's happening. A few years ago a had a 5 hp condensing unit on a walk in cooler that kept going off on oil failure[copeland semi]. The unit did not have a current sensing relay. The circuit breaker feeding power to the unit in the panel was a Square D bolt on type and the bolts that secure the breaker to the buss bar were loose causing the compressor motor to intermittently single phase and shut off on it's internal protector but the compressor contactor was stil pulled in. In that scenario, the oil failure control senses 0 psi oil pressure, goes thru it's time delay and locks out. The same thing happens with a marginal compressor contactor that occasionally single phases the compressor motor causing it to shut off of it's internal protector. If your compressor is overloaded and shutting off on the internal protector, you might have to consider a crankcase pressure regulating valve. Also, Copeland says that some time delay relays leak thru and can cause nuisance trips on Sentroic Oil Failure Controls. The current sensing relay is Johnson Controls # R10A. You can get application engineering bulletins about oil failure controls at Copeland's website, http://www.copeland-corp.com You'll have to register if you've never used the site before [it's free].
08-07-2004, 02:11 AM
Is this unit a dedicated 100% outside air machine? What leaving dewpoint temp is this machine designed for with Florida's thick summer air? What type of control turns on the refrigeration, and where is it mounted?
All of Florida has a summer 79 to 80* Wet Bulb design temp and it requires almost 4 times the Btu requirement for each 400 CFM of 100% outside air. This gives us our "neutral air" leaving dewpoint of 48 to 50*. This is a good range because at 70* DB, the air is at 45% to 48% RH.
If your unit is using 100% outside air and is 144,000 BTU's, then to have a leaving DP of 48*, the air would have to be reduced to 1,300 cfm. At 50* leaving DP, 1,375 CFM. That's less than the air volume of a 3.5 ton regular A/C fan coil! Do you think your machine is moving air that slow? If it isn't, then proper heat removal (latent/sensible ratio) is not being done. I would expect to see no more than .75 horsepower on the blower motor for this air volume and total static pressure. What is the HP of your motor?
With that much refrigeration capacity, and reduced airflow, the pressure in the evaporator coil must be controlled to maintain above freezing temps under lower load conditions. Usually a hot gas bypass valve feeds hot gas into the low side to maintain evaporation above 32*.
Placement of the hot gas discharge into the suction after the evaporator coils results in reduced velocity in the coils to properly return oil. This may be your oil problem.
Introduction of hot gas bypass into the distribution nozzle before the evaporator coils maintains more of a constant velocity through the coils insuring a better chance of oil return. Doing it this way also insures mixing of hot gas with saturated refrigerant, making the txv react to maintain total suction superheat thereby protecting the compressor from being overtemped by excessive direct hot gas bypass.
If your compressor(s) are staged and headered from a single coil suction circuit, or if they have unloaders, you may face the same problem with oil return.
Does the blower operate continuously? If so, then the refrigeration equipment should run non-stop (especially this time of year) to maintain leaving dewpoint and dry bulb temp requirements inside. Where is the condenser for the refrigeration circuit? (In or out of the airflow?)
The only thing that has me is the oil foaming at 25* superheat. If an unloader is cycling, the oil may foam each time the compressor loads up due to reduced crankcase pressure. Is the body temp of the compressor below the heads cool, warm, or hot?
Let me know.
I recently had a florist walk-in chill box unit that kept tripping the Sentroic oil failure control on a copeland compressor. My data logger didn't reveal any problems. I changed the sensor and cable and I haven't had a problem since. You might want to consider that before changing out the unit.
08-08-2004, 05:36 AM
i ran into this type of problem b4. Tyler freezer w/remote condensing unit keep shutting down on oil pressure. turn out to be the float inside the oil seperator
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