Changing fuel filters and some interesting results

Today I changed all the fuel filters on the boat - all eleven of them.  That's a lot of filters, and it's one of my least favorite jobs, mostly because it's so messy.  You might wonder how I came to have so many filters, and I have to admit I was a bit surprised myself.  Here's the body count:

- Dual Racors before the main engine
- Dual on-engine filters on the main Deere 6090
- Single Racor before the wing engine
- Dual on-engine filters on the Deere 4045 wing engine
- Single Racor before the NorthernLights generator
- Single on-engine filter on NL generator
- Single Racor for fuel transfer system
- Single spin-on filter for the diesel heater.

I used a 5 gal pail to collect the waste fuel, filters, seals, etc., and when I was done the bucket was completely full of spent filters

Bucket of spent filters, eleven in total

 While examining the Racor filters, I noticed something very curious that I can't explain.  Maybe someone knows what's going on here.  But first, a little background on my fuel system.  This is my annual filter change, so these filters have been in place for about a year, and have supplied fuel for about 500 hrs of run time.  All of the fuel that gets consumed first gets transferred into a day tank by way of the transfer pump Racor.  So everything gets one pass through that 30 micron filter.  Here it is below, and it looks pretty good

Filter from fuel transfer system.  Looks pretty good.

From the day tank, fuel is drawn out by the various consumers, filtered again by other filters, with some portion returned to the day tank only to get cycled through again.

Below is the filter for the main engine.  Note that it is significantly more fouled than the transfer filter.  But why?

One possible explanation is that the engine returns a large portion of fuel back to the day tank, the fuel actually gets filtered over and over again.  But after one pass through the transfer filter, and one pass through this filter, where is all the crap coming from that gets collected in subsequent circulation?

Another possible explanation lies with heat, and we were just discussing this on the Nordhavn Owners Group.  Deere and Lugger engines (the engines most widely used on Nordhavns) are very tolerant of hot fuel (over 200F), so they almost never have fuel coolers.  In contrast, Cummins engines require that the fuel be kept below something like 130F, and are all equipped with fuel coolers.  The fuel gets hot, by the way, as it circulates through the engine and is returned to the day tank.  Newer common rail engines heat it up even more when it is pressurized.

One interesting comment from the Nordhavn discussion was that hotter fuel tends to release more asphaltine which are small tar-like particles.  I suspect that's what's going on here, but don't know for sure.  Anyone know?

Main engine filter looks much worse, but why?