[ heat transfer ]
Forced-draft AFC
directs the air flow . Usually , the air blows upwards through the tube bundles , cooling the hot liquid . But the fans can be configured to forced or induced draft , depending on whether the air is pushed or pulled through the tube bundle . Large-size industrial AFCs offer a great design freedom in terms of plot configurations , header types , and project specifications that need to be followed . A big advantage is that no water is used . As a rule of thumb , the temperature approach in an AFC is 10 ° C . By using special tube technologies such as Groovy-Fin ® and DIESTA ® the temperature approach can in some cases be pushed to 7 ° C , but it is not advisable to use such low values in the initial project development .
As an example , if a project designs the system for maximum 30 ° C dry-bulb temperature and Groovy fins are used with 7 ° C approach temperature , the closed loop can be cooled down to 37 ° C as a minimum . Temperature approach in a plate and frame exchanger is 2 ° C . The resulting minimum process temperature is therefore 39 ° C .
In regions where water is available , however , open cooling towers can be selected . Water that is sprayed down in the tower is brought in contact with air , which is pulled through the tower fill material by means of a large fan-motor combination . A very small portion of water is evaporated , causing the remaining water to cool down . A big advantage of a cooling tower are the much lower temperatures it can reach compared to dry cooling . In the above-given example , a dry-bulb temperature of 30 ° C was assumed , which is a typical value for Western-European locations . In such locations , however , the wet-bulb temperature can be as low as 21 ° C . A rule of thumb for cooling towers is a 4 ° C approach to wet-bulb temperature . In this case , the cooling water can be cooled down to 25 ° C . An approach of 2 ° C must be added for the closed loop as a plate and frame heat exchanger is used to transfer heat from the closed loop to the open cooling tower . The closed loop is cooled down to 27 ° C . However , another 2 ° C must be added for the plate and frame exchanger in the
Hydrogen Tech World | Issue 6 | October 2022 15