Oil Cooling - A Deeper Look - Verus Engineering

This will be a discussion for oil cooling, in which there are two main ways to cool oil.  Engine oil, transmission oil, hydraulic oil, power steering oil, differential oil, and even shock oil can and will need cooling.  We will focus on engine oil cooling in this article but the two cooling methods apply to any heat rejection application, each with their positives and negatives.  There is not a lot of information regarding either of these methods, the positives, the negatives, or why sometimes they work and other times they do not.  We want to help bring light to this somewhat “black art” of cooling the engine oil.  The information we are pulling from is gathered from over decades of experience as engineers for the world’s leading cooling system supplier for motorsports.  Working with ALMS, to Tudor World Series, to NASCAR, IndyCar, F1 and Trophy Trucks, we have firsthand cooling experience at the pinnacle of multiple motorsports ventures.

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Why does Engine oil heat up?  Engine oil serves two functions, to lubricate the engine and to remove heat from components that coolant cannot (IE under the piston, rods, crankshaft, cams, etc).  As a result, engine oil heats up from both friction and internal combustion increasing temperatures of the engine.

Typical Oil Temperatures:

Before diving into how to cool the oil, we feel it necessary to express the large misconception with oil temperature that seems rampant within the aftermarket community and even some race series.  Engine oil temperature IS NOT engine coolant temperature.  Coolant temperature in passenger cars is generally kept between 180-215 degrees Fahrenheit (85-100 Celsius) with some reaching up to 230 (110 C) safely.  Some race series run coolant temperatures up to 260 degrees Fahrenheit (127 C); however, these engines are designed to handle this without distorting the block or cylinder head. Engine oil, on the other hand, is expected to reach much higher temperatures!  Anyone telling you otherwise should be questioned intently.  Engine oil needs to reach *at least* 100 degrees C (212 degrees F) to burn off condensation (water) build-up within the engine *which is perfectly normal and happens in every single engine*.  If oil does not reach this temperature, the oil is unable to do its job to the best of its abilities and increased engine wear will result.  It is a wise assumption to believe oil temperature sampling is anywhere between 85-95% of the hottest points in the engine, which means engine oil needs to be 85-95 degrees Celsius to burn off condensation.

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So what temperature can you run in your car/engine?

That is quite a loaded question and near impossible to answer without a lot of testing.  We can give it our best shot, though.  Most passenger cars are perfectly fine with oil temps up to 240-260 degrees F *utilizing the OEM recommended oil weight*, with some being designed to handle temps up to 315 Degrees F and higher!  How can this be?  Standard oil these days have flash temps well over 200 C (400 degrees F), and as long as there is sufficient oil pressure, the oil does not care what temperature it is at.  That being said, utilizing an OEM engine and OEM clearances, we would suggest sticking to the OEM oil weight up to around (240-250 Deg. F).  If it makes you feel safer, run a bit thicker oil but with thicker oil, comes increased engine wear at cooler temperatures and increased heat into the oil through more friction and less flow (flow and pressure are inverse, as you increase pressure, you decrease flow).  Above this temperature, we would recommend increasing the hot temperature weight to ensure sufficient oil pressure.  Due to each engine having different optimum operating conditions, we cannot recommend a pressure/RPM to shoot for.

How can we get rid of this heat?

There are two ways to reduce the heat that is transferred into the oil.  Oil to water (or O2W for short) transfers the oil’s heat into the engine’s coolant system through a unit typically called a heat exchanger.  Another route is oil to air (or O2A for short), which transfers the oil’s heat through an exchange with airflow through a unit typically called an oil cooler.  Both of these systems can be implemented incorrectly and work poorly, both of these systems can be implemented correctly and work great.  Individual applications tend to favor a certain method over the other but ultimately that decision should be an end user choice as each person has his or her own goals, objectives, and ideals.  Both can and will work great when properly executed.

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Art Montemayor 18 Jul

Ayche:

Welded plate heat exchangers (also called brazed heat exchangers) are very specialized and unique exchangers that are employed only when one has no other option with regards to potential leaks or spills. I am confounded by your statement that you have found no limitations or major disadvantages in applying them.

Some advantages of using welded exchangers are:

• There are no leaks. This resolves the issue of handling toxic or hazardous fluids;
• They are very compact and take up less space with a small footprint;
• There is presumably no maintenance;
• They tend to be more efficient (with certain caveats).

Some disadvantages are:

• If your fluid gets contaminated and plugs the exchanger, you are in deep trouble;
• You must ensure that both fluids are very clean – always;
• They are limited in their ability to take thermal stresses because of an inability to have expansion joint capabilities; they are rigidly welded together with little or no ability to expand;
• Some composition changes in the two flow streams may give problems because one cannot change or modify the internals;
• They tend to be more expensive; their design is special and usually proprietary;
• If there is corrosion or erosion within the unit, you will not be aware of it until performance drops and the matter can only be resolved by total replacement.
• Some designs cannot handle 2-phase development inside the unit, so any condensation taking place would give problems in performance;

There may be other advantages / disadvantages, but I will wait for other, more experienced and capable members to contribute their comments. The main point here is that there ARE DISADVANTAGES – enough of them to make this type of exchanger a “minority group” member in the heat exchanger realm. I have never heard of one being applied within an oil refinery, for example.

Chris Haslego 18 Jul

O.K., full disclosure. I'll discuss Alfa Laval's technology on the subject because I worked there for many years and know their technology the best, but there are others too.

Art has discussed the classical brazed plate heat exchanger in detail for you above. "Welded Plate Heat Exchangers" is a relatively broad terms and as you've no doubt found, there are several different configurations and options in this class. So, I'll speak from some of my experience.

Most often, welded plate heat exchangers are not accessible for mechanical cleaning (such as the brazed units that Art discusses). To be sure, if you're going to install one of these any where and there is any chance of solids...better install a strainer or a low cost heat exchanger (as brazed units usually are). Many of the inherent mechanical limitations of the brazed units have been addressed in this type of technology:

Alfa Laval AlfaRex

I've applied this very unit in many applications over the years. It has to be justified based on it's cost, but if the criteria are:

No or low solids present (can be reasonable handled by a strainer)
Significant pressure / temperature / operation cycling is expected
High temperature or pressure
Fluids incompatible with gaskets
Flows above a standard brazed unit (these units are available in 4" and 8" connections)

The mechanical change that were made with these particular units was that instead of being welded along the x, y, and z axes, each plate is only welded onto the plate underneath it (only x and y directions). The result is an extremely robust plate pack that acts something like an accordion. I saw one of these burst tested and the head of the tightening bolts was the first part to fail (rather than a plate or one of the laser welds).

The AlfaRex is one of the most robust plate heat exchangers that you'll find. You can hit it with pressure and temperature cycles over and over (many, many times) before it will eventually fail (I've seen them installed for many years in such applications).

Moving on to another class of welded plate heat exchangers, we have what I'll call the welded block style. Alfa Laval's trade name for this type of unit is the Compabloc. Over the past 5-8 years, Alfa Laval has indeed begun installing these types of units in refineries all over the world as a means of maximizing energy recovery.

You can read more about this unit here:

Compabloc

The welded block type unit utilizes lined, carbon steel panels around a welded "heart" of plates that are welded together. These units are welded in all three dimensions meaning that these types of units can be more susceptible to cycling operations. The significant advantage of these welded units is the ability to open them and access the plate "heart" for mechanical cleaning. Also, if a plate were to fail, individual channels can be blocked off much like you would plug a tube in a shell and tube unit.

These block type heat exchangers are among one of my favorites because of their versatility. They're excellent at liquid-liquid heat transfer (especially if one of the fluids is viscous) and they also make very good condensers and reboilers (in relatively clean services). Attached is an article that I authored several years ago on why they make excellent condensers.

There are other welded type plate heat exchangers, but these are two of my favorites....but they're very different, each with their own strengths.

Attached Files

•  HCPCondensing.pdf   717.24KB   939 downloads