... Is condensation really a concern inside the enclosure?... I get condensation on top of my car but not inside of my car....is there enough volume inside the 50X50X10 enclosure to create condensation? ...
The question is not "inside" vs. "outside" but whether a surface's temperature falls below the air's dewpoint. In this example, the car's roof cools to a temperature below the ambient air temperature (and dewpoint) because it's radiating heat to the deep night sky. On nights with heavy, low cloud cover, or if you park in the shade overnight, the deep sky isn't visible; the radiant heat transfer isn't available and condensation/frost will not occur.
(yes, "parking in the shade at night" sounds goofy, but the physics works even if the language doesn't)
The winter concern is not only freezing, but also the fact that ... a lot of the heat generated by the heater will escape out of the vents. My concern is that this will damage the heater after about a month or two of this...
Not a concern. That's what heaters do; it's no different than a baseboard heater putting heat into a room, which then escapes out the walls, roof, windows & doors.
It's possible to eliminate much of this (and unnecessary heater-energy consumption) by placing both the fans and exhaust vents at the bottom and adding a baffle between the two, so that the air must be forced up & over the baffle before being exhausted.
Something else I forgot to mention: The air path should pass over the heater first, then the PLC, then the humidistat & thermostat(s).
How effective will the forced ventilation fan be at preventing condensation if they are on above 80F? Could I then potentially ignore dew points above 80F since the fans are on above 80F?
Forced ventilation will potentially make things worse, not better, by bringing in unconditioned air from outside and lowering the temperature. But if the fan remains off while the in-cabinet temperature is maybe 10
oF higher then the highest dewpoint that will ever be encountered, it's not a concern.
... How can I prevent [short-cycling oscillation] from happening? Should I let the ventilation fans overlap with the heaters? For example, the ventilation fans come on at 80 F, and the heater shuts off at 90 F. Would this overlap help, or would I have the same potential for short cycling?
You're on the right track, but in the wrong lane. The way to prevent it is with a dead zone in which neither one turns on. For example, heater off at (or below) 80
oF; fans on at (or above) 90
oF.
... This would cover me for pretty high Temp/RH combinations (90F and 95%) ...
The highest temperature and the highest relative humidity do not occur at the same time. You can design for each condition independently.
... If the hygrostat + heater solution only lets the enclosure cool down if the %RH below 50%, but doesn't remove any water, will it ever let the enclosure cool down?
1) Raising the temperature will lower the relative humidity, even if it doesn't remove any water. When the RH is under control, the temperature will find a corresponding equilibrium, not continue rising.
2) Do you care? As long as the PLC temperature is within limits, what's the harm in it being warmer than ambient?