How much liquid are you cooling? If you're using 50 gallons of water to cool a 5-6 gallon batch in 30 minutes, you're not using the exchanger correctly. I can cool over 6 gallons in lots less than 15 minutes with about 10 gallons of water with an immersion chiller and our cooler tap water temps in Colorado (not necessarily the case in summer). Are you using any type of a coil (in ice bath) to help cool the water running through your exchanger? Pump? Recirc? Valves to slow flow? You could be pushing to fast through your plate. There's ways to get colder liquid through your exchanger or immersion chiller.