Cooling water systems are responsible for taking out energy from the process. They are very important and can impact production.
There are some types of cooling water systems, in the Cooling Tower (CT) type air is used to decrease water temperature.
As Chemical Process Engineer you need to give some info to the Supplier, or to the Mechanical Engineer, to the design and selection of a CT to our process. Recirculation flow, temperature rising (or range) and air wet bulb as a minimum. Info about hot day, cold day, highest recorded temperature and lowest recorded temperature helps to evaluate equipment performance in multiple cases. Usually dry bulb and barometric pressure are also informed.
Wet bulb is the starting point of your process design once cold water temperature (cwt) is based on the approach. Calculated as Water Cold Temperature – Wet Bulb. Based on economic reason approach ranges of 3 to 6 oC.
A good reference for local data is www.weatherbase.com. However, you need to do some calculations to define wet-bulb temperature. Another good reference for local data is ASHRAE Fundamentals Handbook. ASHRAE uses a statistical method to define the design wet bulb.
Air Heater Exchanger is an option when freshwater is not available or its use is very expensive.
In the Air Cooling Heater Exchanger recirculating water is cooled without contact with air. Air is fan across the coil bundle (finned-tube design) where hot water inside tubes is cooled in a heater exchanger. In this kind of system, Chemical Process Engineer shall consider local air dry-bulb (Cooling Tower uses air wet bulb).
Click in the video to see air cooler animation.
Based on API 661/ISO 13706 for critical application design dry bulb should cover 99.5% of time and to non-critical application, dry bulb should be the highest temperature between 95.5% cover and 99.5% cover minus 4 oC.
Process data for equipment includes recirculation flow, design dry bulb, water temperature differential (or rising temperature) and approach to dry bulb (usually 8 to 14 oC).
Note that for the same design basis Air Heater Exchanger will produce cooling water with higher temperature when compared to Cooling Tower. First, because the dry bulb is always higher than the wet bulb and second because there is no latent heat/mass transfer effects between water and air.
