This section is by no means complete or as comprehensive as my previous investigations.
Treat it as a quick'n'dirty cheat sheet on cooling a modular engine post Space DLC.
I'll just put my findings and fixes in here so folks can have properly cooled engines again.

Direct Seawater Cooling is no longer the King, as of now it appears that the crown goes to the 5x5 fan radiator. I didn't extensively test the other cooling components, I'm assuming they're placed relative to each other in terms of cooling efficiency as pre update, maybe the air/liquid heat exchangers are slightly better now, due to higher air/gas flowrates compared to liquids.

Coolant tanks are back!
Mostly due to the fact that prefab tanks are pressurized now, and spawn in with 60 ATM, you can push the radiators coolant flowrate above 350l/s, making for some efficient cooling.
Simply put a medium prefab tank between coolant manifold and radiator input pump as seen in the screenshot to achieve such a high flowrate:


This gives a nice pressure difference between input and output of the coolant manifold, it's also possible to simply stack multiple of those pressurized radiator loops without breaking the flowrate or flow at all, which has been the case in previous versions of Stormworks.

Here's the radiator in action, capable of cooling a 4cyl 5x5 supercharged at 0.85 fuel and 1.0 air throttle at ~10RPS:


I have stacked 8 of these radiator loops, capable of keeping the above mentioned engine at ~80° with full load while ~25km south of arid island with an outside air temperature of 30°. Pretty much overkill, 5-6 loops would be able to handle it as well.

5x5 Radiator Setup (tileable):
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=3114294781

I'll keep brief updates to this section in case any further game update will change things again.

As of one week after the Space DLC launch, steam and fluid systems (cooling etc.) seem to be erratic at most. It's best to consider this guide as outdated.
It's rather unlikely for me to update it since I didn't touch the game for almost half a year now, hopefully it will still be of help to some folks playing older versions of SW.


I've messed about with modular engines since they became available to stable branch and have seen them go through a variety of changes (especially patch 1.2.3, cutting fluid flowrate in half, with no mention in the changelogs).

They are in a pretty good place right now, if you utilize tremendous amounts of engine power you need tremendous amounts of cooling. Luckily modular engines are extremely powerful, even more so when supercharged and excessive cooling is only needed for the most powerhungry builds.

My only gripe with the cooling (besides bugs) is that the 3x3 and 5x5 radiators are more efficient than the 2x2 and 5x5 Liquid/Liquid heat exchangers, as you will see in the charts provided.

Vehicles that are putting their engines under constant load (boats, generators, aircraft) need more cooling than vehicles with a variance on engine load (land vehicles which don't have to constantly fight drag from air/water).

For getting an efficient cooling system it is mandatory to have a high flowrate inside the cooling components.
The flowrate can be seen with the advanced tooltip, enabled in the general settings.

Best case circuit for modular engines:
Coolant manifold -> Pump -> Heat exchanger/radiator -> Pump -> Coolant manifold.

For prefab diesel engines or condensers the circuit is almost the same except for the coolant manifold.

If the flowrate is heavily fluctuating it`s usually an indicator of a component placement bug, where the order of placement can affect the final flowrate. Simply delete the pumps+radiators and place them again in a different order, this usually fixes the bug.

Here's a link to the bug:
https://geometa.co.uk/support/stormworks/2468
According to the developers response it's not that big of an issue, despite breaking clutch functionality, heavily reducing heat transfer for cooling components and making modular engines completely refuse to work, if the bug occurs.

Having a single pump on inlet and another pump on the outlet side is the best practice, the flowrate tooltip should show a stable, non fluctuating symmetrical flowrate for input and output.

Flowrate will max out around 75l/s, some of the screenshots were made with the pre 1.2.3 fluid system, which allowed for a maximum of 150l/s and is no longer possible in game.

There doesn't seem to be any clear distinction between large and small electrical pumps in terms of maximum flowrate, so use what best fits the purpose. Impellers are rather impractical and either consume too much electricity from motors or engine output if not properly geared down.

The coolant manifold is your access point to a modular engines coolant reservoire.
It displays the entire coolant volume for all cooling components connected to the common engine manifolds.

Imagine a custom tank with an input and output port.
The coolant manifold is essentially an input/output port at the same time, while all cylinders, radiators and pumps are the tank itself, holding all the coolant.

You don't have to connect both sides of the coolant manifold, you can pump coolant out one side just to pump it through a radiator and back into an entirely different coolant manifold.

It is also possible to use the engine manifolds to extend towards an area where you have plenty of space for cooling components and simply put your coolant manifolds right next to your pumps and radiators.

Engines generate heat by the amount of fuel burned. RPS doesn't play a big role in that, since you can rev an engine to high RPS, if the load is low you won't need to use much throttle to reach those RPS and in turn not burn that much fuel.

When your engine is powering a generator with gearboxes increasing generator RPS, your engine load will be high, throttle will be sitting at 1 and the fuel consumption will be measured in barrels per second, then you're in for a warm welcome.

Engines cooled with seawater that reach more than 100° will now experience scaling.
This means that the coolant will form salt scales/corrode the insides of the pipes, causing an increasing loss in heat transfer.
It's currently untested if long term usage of seawater cooling below 100° will scale up the engine at a slower rate.
The coolant manifold will show you the current scale percentage, at 0% everything is in order. Anything above causes reduced heat transfer and will most likely result in an overheating engine, over time.

The most efficient setup in terms of piping has a single heat exchanger per coolant manifold.
If you add multiple heat exchangers to a single coolant manifold, the second in line will have less than 50% flowrate of the first one, which makes running heat exchangers in series a terrible idea.
Flowrate on a single heat exchanger:


Heavily reduced flowrate on the second heat exchanger when piped in series:


Use engine manifolds to reach your heat exchangers and place coolant manifolds in a way that leaves the least amount of pipes between coolant manifold, pumps and heat exchangers.

This is a basic example of how an optimal layout looks like:


Of course you might need more than one heat exchanger, then you need to place multiple coolant manifolds on your engine manifolds, like so:


These are just examples of the layout, you will of course run into space constraints and can adapt to that, a more compact layout would look like that:

It is important to keep as few pipes between the components as necessary.

You do not need fresh water tanks for radiators.
I've done some further testing on my 32m workboat catamaran with a total of 24 radiators and 48 pumps.
The complete cooling circuit is holding 6004l per engine.

Adding coolant to the circuit is not recommended, the circuit already spawns with plenty of coolant.

Adding coolant breaks fluid flow and clogs up the circuit for 50% of all radiators in my test setup, severly hampering cooling efficiency.

Do not add coolant to your cooling circuits.

With the major oil update, rooms now have their own temperature.
If you have a furnace in the same room as your cooling components, they will have a hard time fighting excessively hot furnaces, due to the room heating up over time.
It is also recommended to watch your ambient temperature outside of any rooms, if you have a reactor or furnace on your ship, since the outside tends to heat up faster than the room the furnace is in, rendering all cooling components nearby inoperable, especially with excessively hot furnaces/reactors.

Liquid/liquid heat exchangers have different sides, named A and B. Make sure the B connectors are connected to the engines coolant manifold and the A side to sea water. Other way around didn't provide any cooling during my test runs.

When using Liquid/liquid heat exchangers, make sure the seawater outlet is above the waterline. Otherwise the pumps will have to fight outside pressure, leading to pressure fluctuations. Thanks @Captain for bringing that up in the comment section.

Ambient temperature also plays into cooling efficiency. Cooling in the arctic doesn't require as much effort as further south. I've observed a maximum temperature of around 40° roughly 20km south of sawyer islands. Note that seasons also are a thing and ambient temperatures are affected by that.

Naturally aspirated modular engines are rather tame in terms of heat generation. Supercharging them can increase their power output at least by a factor of 2.3, which increases air and fuel consumption and also results in a bigger heat generation. If you have a supercharged engine then there's no way around using fan radiators, since these are the most efficient cooling components.

If you have the space it's always worth going with more cylinders or a bigger engine.
More engine power means you can run the engine with less throttle to reach your desired speeds, which allows for cooler running engines.
Take a look at my Dieselpunk train on the workshop, which is using a 8cyl 5x5, which is absolutely oversized and does not need any cooling at all.

Comparing different cooling setups was done on a hydrofoil testbed with a 6cyl 1x1 supercharged.
With the test setup all different cooling setups were able to prevent the engine from overheating.
I also gathered data running without any cooling components at all, as a reference.

Testing procedure:
Throttle was fixed at 0.75
After starting the engine the boat would move at a constant 92km/h with no steering inputs or other events that might influence RPS to provide a stable heat generation throughout the tests.
Above 25° the cooling components were activated and above 30° I gathered temperature and change in temperature/s by using the CSV logger made by Lupus the Canine (link in the last section of this guide).
The test continued until the temperature reached 100° or the temperature/s dropped below 0.05 with the predicted equilibrium temperature being below overheat temperature of 115°.

I've tested the following components:

• 1x2 Air/Liquid
  
• 5x3 Air/Liquid
  
• 2x2 Liquid/Liquid
  
• 5x5 Liquid/Liquid
  
• Fluid Heat Radiator
  
• Fluid Heat Sink
  
• 3x3 Radiator
  
• 5x5 Radiator

Tests:
The charts show temperature on the x axis and degrees per second on the y axis.









Here's a more recent chart where I tested various cooling methods and also if air used as a coolant would have any effect (it doesn't):

Vertical axis shows degrees per second, horizontal axis is time.
The lowest line is direct seawater cooling, being by far the most efficient cooling method currently.

Engines require either ocean water or fresh water as coolant, otherwise no cooling is happening.
Cooling the air for the air intakes doesn't affect engine heat generation.
The only purpose for these so far is either cooling steam, if, for some reason you need to run your boiler hotter than 110°, or cooling the exhaust. Exhaust will be black if above 115° when exiting the pipe and will be gray when below 115°.

The less silly brother of the Air/Air heat exchanger. According to the tooltip it's an intercooler, so it should be used as such.
I've set up a test rig with 2 7cyl 5x5 engines, both with 7x 5x5 fan radiators and 10 liquid/liquid heat exchangers each.

One engine also had an additional 9x5 Air/Liquid intercooler, using air for the air side and engine coolant for the B side, straight from the coolant manifold.
After a good while the engines hit their final temperatures, with the engine that received an additional 9x5 air/liquid intercooler running 0.1° less hot, settling at 108.3°. The regular engine hit 108.4°. Not the difference you'd expect from a 9x5 sized, 225 mass block.

This one can also be used to intercool steam before sending it to the condenser. Best way to set up would be having steam go through the air connections, and hook the liquid side to either sea water or a radiator.

Another usecase for the Air/Liquid heat exchangers are aircrafts. Using 2 air scoops on the air intake facing forward, and 2 air scoops on the output facing backwards provides significant cooling and seems to be plenty for most applications. Though I didn't do extensive testing for this configuration, it always seemed to work a treat keeping aircraft engines below 115°.

Now for the conclusion, first the chart with all accumulated data:


And the final rankings from most efficient to least efficient, left to right:


Surprisingly the Liquid/Liquid exchangers are less effective than the radiators, with the 3x3 radiator being a very close match to the 5x5 Liquid/Liquid heat exchanger.
Direct seawater cooling still wins the cake and is far more efficient than any other means.

This concludes this guide, if you think I could add to it or have other suggestions feel free to leave them in the comments.

CSV Logger by Lupus the Canine:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2408195283

Simple AFR Controller by Adester:
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2386277437
You can use this one for supercharging on the fly. It also allows to set a fixed stoichiometric ratio. Using 0.2 will provide you with the most amount of power/fuel while also being the least amount of heat/fuel consumed. This also helps with cooling your engine.