What Are Economical Methods to Cool Down Mineral Oil in Server Tanks?

[berkeman]

It is not surprising there have been so many suggestions. The OP has not given a clue as to the system's environment.

He did say this about mid-way down page #1 of the thread:

The weather is dry and can reach up to 40°C during summer. The building is 4 story and access to the roof is possible but it's not necessary since the hot air will be dumped into open air (through the yard).

 

[Lnewqban]

I did something similar. I created a "tunnel" using tarp; at one end I put a commercial Evaporative cooler (15000 m3/h blowing power) and at the other end I placed the servers with their fans removed. the problem was that the servers' boards are really dense and don't have that much space between them. so the air didn't cool them all properly and some of the boards reached critical temperature.
...
I don't know maybe I did something wrong but that didn't work out for me. the length of the "wind tunnel" was 1.5 meters and had a diameter of 1.5 meters.

That is a huge amount of air flow (and dust), but only a fraction of it reached the surface of the heat sources.

It is unlikely that your servers are actually and simultaneously generating the amount of heat that you have assumed.
That could be your case, but in my experience, server rooms needing more than 3.5-ton of air cooling are not typical.
You will greatly oversize your cooling solution if you go by the watts that the nameplate of each server shows.

You could calculate the actual amount of heat to be removed in two ways:
1) Measuring the number of amps entering the room in certain period of time.
2) Measuring the time needed for the cooling oil to reach certain delta temperature (keeping enough forced flow of oil in contact with the heat generating surfaces is very important).

Performing several tests at the peak demand and worst environmental conditions will give you a solid number to work with.
With that number in hand, you will be able to present the problem to your superiors and to make better decisions about the more economic and practical solution.

Either you will need to resort to refrigeration (air cooled oil chiller recommended) or to evaporative cooling will make a significant financial difference.
Don't fight this problem alone, involve the persons in charge of making financial decisions and specialized technical help able to design an efficient solution.