300 cfm per ton?

Can the system handle the low load without freezing up?

There are speed drives available.

wanna cut the airflow back 25, 30%? That’s a big drop

Check out the Daikin Quaternity, they have two coils and can heat and cool simultaneously to maximize dehumidification. Although for some reason they are not very popular, I've never encountered anyone who had one installed. They also may have limited availability, I don't think they've come out with a new model to comply with the new refrigerant requirements.

That said, when last I looked the Quaternity I came to the conclusion that you're better off from an energy use perspective running a two-stage dehumidifier like the Quest MCORR. Product page is:

https://www.questclimate.com/m-corr-technology/

There's a discussion here: https://www.greenbuildingadvisor.com/question/quest-m-corr-dehumidifier-technology

I was told you can go down to 250/ton from the usual 400-450. But the temp your maintaining the space at has a lot to do with that. Get too low and get freeze ups

This is frequently done in lab spaces or in some process enviroments.

Your going to lower the evap temp by moving less air. If you have high humidity that is a load increase so it may not affect things much.

Normal evap temp is about 40 so only go as low as you can to keep it above freezing. When the suction pressure converted to temp gets to 32 your done although they say with the air moving it won't start to frost until about 25. depends on the moisture content.

Some units (I think Trane) have a discharge gas coil for reheat.

@ChrisJ Have a great book on humidity control Chris if you wan't it PM your address. i don't need it anymore.

or put an ecm motor on the blower that you can fiddle with to your heart's content.

A Rawal valve is basically hot gas bypass in a kit. It would let you turn the air down nearly as much as you want and still keep the suction pressure up. I've installed them in splits before, I wouldn't put all the service valves in again, there's just not enough room. Maybe one in the hot gas line so you can turn it off. Bring all your takeoffs off the top of the pipes so the oil stays where it should.

the Rawal device is for limiting capacity. Is it proper in this situation?

If it were matched to the load you would be running long cycles and getting good dehumidification so you do want to reduce capacity. A standalone or duct connected dehumidifier might do it too.

Don't know what temp and humidity you want to maintain but we designed some with 70 sensible and 50%RH which is a dew point of about 50 deg if I remember right.

I remember a chilled water job we did with a new Carrier Chiller at Eveready Battery. We had the CW temp at 44 supply and 54 return typical chiller temps and we couldn't get the RH down. I installed it but didn't design it.

My boss had all these people running around who were "real engineers" wondering why it wouldn't work. They proposed plastic coated ceiling tiles and epoxy on the floor to combat humidity infiltration. I sat on the floor looking at my psych chart and checking the coil outlet temps. I realized the the whole coil had to be below 50 degrees and it wasn't with a 54 water return.

I went upstairs to the chiller and set the supply to 38 (the lowest temp Carrier would approve without glycol) By the time I got back to the space the humidity was gone. Humidity moves very fast.

is this a commercial building? If it is the is the wrong program. The fact you have equipment generating steam is also an issue.

Need a load calculation to see if the equipment can do the job before anything is done

Manual CS

In just about all situations a colder coil means more dehumidification. That stops being true when the coil gets above the dew point of the air, and there is a practical limit that if the coil is below freezing it will ice up and block the flow of air and everything shuts down.

Coil temperature is determined by the ratio of air flow to refrigerant, the less air the colder the coil. While you can easily control the flow of air, you have to realize that the compressor control logic is going to modulate the flow of coolant as well. Depending on the system there might be a TXV, modulation of the compressor or a temperature sensor on the coil.

Note that when performance is measured for SEER ratings only sensible cooling is required. So manufacturers are incentivized to keep their coil temperature up and minimize dehumidification if they want a high SEER rating. Sometimes they compensate by having a "dry" mode that maximized dehumidification.

A separate issue is run time. All other things being equal, a longer run time will lead to more dehumidification. Dehumidification happens when dew forms on the coil and drips off into the drain pan. It takes a few minutes at the start of the cycle for enough dew to form to start dripping. At the end of the cycle, any dew that is still on the coil and hasn't dripped off yet will just evaporate, so the moisture is returned to the air. If the cycle is so short that no condensate is removed, no dehumidification happens. Generally short-cycling is an indication of oversized equipment.

If it is a txv system, won't the txv keep the coil form freezing if you slow the airflow down? I suppose the first thing to do is to measure the evaporator temp. It is oversized on everything but nearly the hottest day…

Maybe exhaust fans at the equipment to take the humidity and heat outside instead of trying to condense it out of the air?

Exhaust fans will not help. You exhaust air and more air infiltrates in putting more load on the system.

Sample of 1, had similar issue at a towhose I lived at many moons ago. Installed a fan speed controller on the blower fan and turned it down till the supply air got to around 40F. Fixed the humidity issue. You do have to keep an eye on it to make sure the coil is not icing up.

I was troubleshooting equipment earlier this week with condensation issue. Outdoor temp was 68F and dewpoint was 65F. If you are in a place like that, adjusting the AC won't help, you'll need a dehumidifier.

Resistance re-heat is very expensive to run, last rest.

isn't this basically what i said
"The needle is controlled by a diaphragm subject to three forces. The evaporator pressure is exerted beneath the diaphragm tending to close the valve. The force of a superheat spring is also exerted beneath the diaphragm in the closing direction. Opposing these two forces is the pressure exerted by the charge in the thermal bulb, which is attached to the suction line at the outlet of the evaporator. "

The one @ChrisJ posted is externally equalized, i think most used for small dx systems are internally equalized.