We have designed an air cooler for a Mineral Oil ISO 68 as per API 661 with the following requirements:
a) 10% overdesign;
b) The cooler shall be provided with two fans. Each fan shall be capable of 100% of the duty requirement. Or provide an air cooler with 3 fans (or maybe six), and it is necessary the full thermal guarantee by using only two fans (or four fans if six fans are provided).

The first requirement (a) is typical for an air cooler but the second condition (b) is a bit strange and difficult to comply.
When we started with the design using Aspentech EDR, we began by designing for the condition of having 3 fans and 1 of them out of service (this is the most restrictive condition for the design), and then rating the air cooler with the tree fans in operation.
To simulate the condition of having only 2 of the 3 fans in operation we use the “Outside Distribution” in EDR with 3 number of elements along the length of the bundle, entering the flow distribution of air as: 50% / 50% /0%.

After getting the right design, we simulate it for natural convection in order to know the quantity of air that passes through the fan that is not in operation and then with this flow, we adjusted the design: the result was 49.5/49.5/1%.
Once the design was finished we asked the fan vendor for quotation, and as the static pressure when the tree fans were operative was very low, it was impossible for him to quote them: the same design of fan should provide when the 3 fans were operating 2 hp and for the second condition (2 fans on & 1 off) 27 hp.

He suggested to operate always with two fans and to implement a maintenance routine that consist in rotating the fan that is out of service in order to guarantee an equal mechanical wear in all of them.

We would like to know if you consider that the way we are using to design the air cooler is right, and if you have gone through a similar case. All the information or experience that you can give us will be very helpful.

Furthermore, we would like to know if you have experience in designing an air cooler with no equal air distribution with Aspentech EDR, or if you consider that there is a better option to use.

Thank you in advance for your cooperation.