Carrier MA units ,Config and Control question.

[fat eddy]

An engineer has specified a control upgrade for several Carrier MA units. These are the mid to late 80’s version of multizones that Carrier made. The upgrade is to include a comparative enthalpy sequence and CO2 control, the enthalpy sequence will choose outdoor enthalpy or return air enthalpy for more efficient operation, although these are normal sequences in control schemes for schools, I am having a problem determining how it could work on these units, the units configuration shows that some of the condenser fans double as exhaust fans during econ operation and they come on when the exhaust damper reaches a predetermined position,

With this configuration, and needing to have the power exhaust on when using outdoor air during mechanical cooling operation, I don’t see how this can work, I can’t come up with anything that will allow this sequence to happen with this unit, basically what I see is,

This unit can never run outside air above a certain minimum position while it is running mechanical. Which also means that it can not react to high CO2 levels either during the mechanical cycle.

Is anyone familiar with this type of unit and this type of problem with the unit?

And how you solved the problem ?

If it is a problem it needs to be brought to their attention in a hurry because the bid package is out and it involves 63 of these units.

[stopro1]

but it sounds like you are confusing very different functions.

min oa is just what it says. while using the same dampers and actuator, it has nothing to do with econo function. it's function is to provide a min fixed oa, or variable oa if used with a co2 sensor in a demand control situation, as you have described in this example. either method should be active only when there is both a running supply blower and in the occupied mode.

an econo is active only when there is a call for cooling and may open the oa damper more than what the min oa position to maintain the req'd mixed air temp. the oa damper will never close below the min pos when in the occ mode, but may in unocc as there should be no min setting as mentioned above.

diff enhalpy control simply determines if oa or ra has less total heat, and chooses to use the one with less for mechanical cooling, not econo mode.

power exhaust should be activated by oa damper position only, no matter if in min pos, econo or mech cooling cycles.

hope this helps.

[fat eddy]

I understand those things,

The problem is that the unit uses the condenser fans for exhaust when it it is in the econ mode, so if I initiate a sequence that opens the OA and exhaust dampers during mechanical cooling, It will try to exhaust, the exhaust air outlet of the damper is located between the condenser fans and the condenser coil, so I'm thinking it will exhaust building air, while letting the condenser starve for air and in turn cause refer problems

follow me ?

It is basically a dual purpose fan and seems like econ and mech were never meant to run at the same time.

You may have needed to have seen the the unit , to follow what I'm talking about.I will look it up on there site and see if they have a pic of it.

[stopro1]

sounds pretty cool, never seen it. a pic would help. but you say in the econo mode, so how does that affect the mech cooling/refer cycle. what exhausts the air in oher situations

[docholiday]

If you're using the economizer fuction, it generally means the outdoor ambient is lower and less airflow through the coil is needed. (much like a motor speed controller)

[fat eddy]

StopPro,
It will affect it because in the CO2 sequence or the comparitive enthalpy sequence it will open the exhaust and OA damper, which wil cause the condenser to move exhaust air, in place of pulling air through the condender coil.

Doc,
Its not the econ sequence that I am worried about, its the comparitive enthalpy and the CO2 sequence which will need to run during mechanical cooling.

[fat eddy]

Here is a pic of it,


http://www.imagewiz.net/usr/hvac/2539_Drawing2.jpg

[Edited by fat eddy on 04-02-2005 at 07:45 PM]

[bradysmill]

Eddie,
I happen to have had experience with several of these units over the years and actually did somewhat like you are describing to one last year at a local elementary school during a renovation to a gym.

The condenser fans are nothing more than power exhausters when the damper position opens to about 30-40% (adjustable.)

When you are done with the changes you want to make, the power exhausters/dampers will be controlled by the co limit setpoint (if I remember codes I think setpoints will be around 800 ppm???) This can be a co monitor that will activate a spst relay that will turn control of the dampers over to a fixed pot (you would install-like a typical honeywell min. pot.)that will be wired in the existing min. damper position circuit as an override (set the co pot to "override" the dampers to a position you think(or experiment with) will vent the room air of co. slowly... preferably without having the power exauster come on.

I have set up many units to run oa and mechanical cooling at the same time. Depending on a lot of variables about the system you have, do not really see a problem there.

NOTE: If the oa temp. is 50 or below, the mech. cooling may have to be locked out.

If its hot out when co trips....txv will adjust superheat(rember you set the opening position of the co trip damper setting.)

Enthalpy and return air sensor will work "together" to determine if oa is to be used only....if economizer is in operation.....theoretically co trip should not happen, if it does, then the dampers would open further to the co min. pot setting.

It may take several attempts during different conditions to set this up right but I can't remember having any problems with it that way.

I think I have this right if I am reading your post right.

[fat eddy]

Brady,

Yes I think you have it right,

My main concern is what will happen if I go to a high or full OA setting, will the air leaving the exhaust interfere with the ability of the condenser fan to pull air across the condenser coil ? The config of the unit leads me to beleive that it will.

[mrhvacmechanic]

ED,
Check with carrier, I belive your other cond fan will handle compressor load (as lond as compartments are isolated) since your oa is below 50F. The fan that is being used for exhaust would probably be locked out below 60F with a oa dick or stat. Your other fan would run on a motor master to maintain head below 50F.So they utilize the fan that would be locked out for your exhaust.

[fat eddy]

MR,

I understand that in economizer there won't be much of a problem, but their sequence calls for conditions that will want a lot of outside air brought air in during high OA temps. Carrier is the one who laid out the control sequence for the unit that the engineer speced.,

What I see is a chance to get an unfair advantage over the rest of the bidders, I'm figuring that I am right about not being able to implement this sequence. So I am thinking about going ahead and bidding it a little low, then getting the job and going back to Carrier and the engineer after the fact and explaining to them that it will not work, this will reduce the re-control labor and parts costs in half, I would expect that they will not rebid it and I will get the job at the same price that they have already accepted because the job is already going to pass a deadline, and I am saving them some embarassment, and will be able to better meet their deadline, than anyone else.

[mrhvacmechanic]

Here in NYC were seeing alot of air side economizers using the cond fans as a dump. Even the indoor packaged air cooled units also. They are putting dampers on the cond intake ducts and pull in cond fans with a damper end swith. Another job we did the engineer used a dwyer static controll to sens space pressure and oped damper with it.

The units used sporlan head master controls for low ambient operation.

As far as OA quantities, that unit better be large enough to keep space cool when using larger warm amounts of air.

Good luck on you bid but be careful, Carrier can afford loses where guys like us can't when bidding.

[Edited by mrhvacmechanic on 04-03-2005 at 09:06 PM]

[stopro1]

Eddie, if the machine is configured as you describe, and exhaust air from the bldg does not get drawn thru the condenser coil by the fan, I would agree that there may be a problem when mech cooling & high oa quantities are are acive.