Processes run on energy
Almost nothing in a plant happens at room temperature. To boil a liquid, drive a reaction or dry a solid you must add energy; to condense a vapor or cool a product you must remove it.
Two blocks handle this: a Heater adds heat, a Cooler removes it. The amount of energy per hour is the duty, measured in kilowatts (kW) — the same unit as a kettle or a car engine.
Heaters add energy, coolers remove it; the rate is the duty, in kW.