Using a PID to control a digital output

Hey guys, here's my situation. I'm rebuilding a panel at work and we're eliminating a couple of temperature controllers in favor of using the PID functionality of the Micrologix 1100 (the panel is in a food plant and is sprayed down daily so we lose these door-mounted controllers a lot). The tunnel is cooled using liquid CO2 and a series of valves. The valves open to cool the tunnel and close when the temperature is met. Since the temperature has to remain as constant as possible, I feel I should use a PID loop instead of a simple high-low limit scheme. My question is what would be the best way to use a PID loop to control a digital output? I'm really inexperienced with PIDs and so far I've only run into controlling an analog output with an analog input.