When running submerged, does a modern military submarine require heat to keep it warm, or does it give off excess heat?
Just a random question that popped into my head on the drive home the other day. Old movies sometimes show submarine crews sweating in light shirts and slacks. Yet these same metal cans are surrounded by cold water.
So which is greater, the waste heat that a submarine produces (engines, electronics, lights, people, etc) or the heat loss to the surrounding water?
Why can’t it both require heat to keep it warm and also lose that heat to the surrounding seawater?
For it to be endothermic, wouldn’t it have to absorb heat from the water? (That seems difficult considering how cold most seawater is.)
Besides, the heat will necessarily have to dissipate into the surroundings at some point, else the sub would be a self-contained energy trap and would get infinitely hot.
I think the OP is basically asking whether modern submarines require heaters in order to keep it comfortable for the inhabitants, or whether they require some clever waste-heat management in order to keep it cool enough to be comfortable.
Nuclear submarines use waste-heat from the reactor to heat the air. And of course they give off some waste heat into the water. If it were the other way around, the submarine would be colder than the (very cold) sea water.
Okay, just some rough calculations. An Ohio class submarine is 170 meters long and 13 meters in diameter. Calling it a cylinder, which it’s not, it would have 7,200 square meters of surface area. Is it double hulled? Beats me, but assume it’s not. The heat transfer coefficient of steel is 25 W/m^2*K. If the sea water were 4 deg C and the inside 25 deg C that’s a delta of 21 K, so figure heat lost to the sea as being 3.8 MW.
It’s got two 45 MW turbines. According to Wiki, marine steam cycle turbines have yet to break the 50% barrier in efficiency, so half of the 90 MW is wasted, most likely as heat. Some noise of course, but the Ohio class is pretty quiet.
So, heat loss through the shell to the sea by conduction, 3.8 MW. Heat wasted by the turbines, 45 MW. I’m calling it exothermic. I made a lot of assumptions, and my math is sloppy but maybe my mistakes will cancel out.
There’s going to be some big ass heat exchangers on board to get rid of the extra heat.
No question - this will be true for any steam plant. (Okay, any condensing steam plant - if you have unlimited fresh water and don’t care about efficiency, you can omit the condenser.)
Can you explain this? I was in the US Navy on submarines for years and I don’t recall anything like this.
We were required to run the air conditioning all the time to provide cooling to the electronics in the forward part of the boat even when we were in the Arctic.
Seawater was around 30F. I’ve even seen 28F. The AC units did not run very well when seawater was that cold and with very little load on them. They would run but you had to throttle the seawater to them which was bad for the valves.
Just curious, but how come the Navy doesn’t use thermal imaging to detect subs instead of sonar? Does it not work underwater?
Another former submariner here…
Modern nuclear submarines definitely need to cool the engine room, and either need to heat or cool the forward compartment(s) where the crew lives, depending on the temperature of the surrounding seawater and the what kind of equipment is in the space.
The A/C units back aft use electrically driven compressors. If the reactor goes down, and the ship rigs for reduced electrical load, the A/C units are shut down. Within about 15 minutes, temperatures in the upper level of the engine room will rise from the normal temperature of about 80-85 deg F to about 115 deg F. Temperatures are usually 10-15 degrees cooler in the lower level of the engine room.
Temperatures in the forward part of the submarine are somewhat dependent on the outside seawater temperature. Some areas (such as the galley and spaces full of electronics) always needed to be cooled with A/C, and some areas need electric heaters in conjunction with the forced-air ventilation system.
Agreed–this is incorrect. Waste heat from the reactor and steam plant does heat up the engine room, but as previously mentioned, large A/C units are needed to counteract the excess heat.
This is true.
The reason why old movies show the crew sweating is because they had huge diesel engines (when operating on the surface) running in enclosed spaces with no A/C. Up forward, they had (primitive) electronics that gave off large amounts of heat, with generally poor ventilation. You also had the crew crammed into small berthing areas, again with poor ventilation.
Modern submarines have ample amounts of electrical power to run A/C units, heaters, and forced-air ventilation to keep the crew comfortable.
Water is a real good transmitter of sound and a real crappy transmitter of IR.
[quote=“Bill_Door, post:5, topic:561152”]
Okay, just some rough calculations. An Ohio class submarine is 170 meters long and 13 meters in diameter. Calling it a cylinder, which it’s not, it would have 7,200 square meters of surface area. Is it double hulled? Beats me, but assume it’s not.were 4 deg C and the inside 25 deg C that’s a delta of 21 K, so figure heat lost to the
[quote]
Soviet subs seem to be double hulled but not US nuclear designs.
I would also guess the laundry, showers and simple space-per-man ratio are all better in modern subs than they were in U-boots and sugar boats. Doubly so on boomers like the Ohio, which are very large boats (cubic volume per man : ~160m^3 on the Ohio, ~35m^3 on a Type VII U-boot).
One must note however that living conditions aboard WW2 US subs were much better than those of German subs : Gato-class subs for example not only had fridges to keep fresh fruit & veggies throughout a long deployment, they also had AC and each crewmember had his own bunk.
Type VIIs (like the one in Das Boot) didn’t have any of that because the comfort of the crew was way down BdU’s priority list. Übermenschen could handle living like sardines in a can that permanently smelled of warm, wet, rusty fart and machine oil, ja ?
They didn’t hot bunk?
Well, also because before the war the Germans were treaty-limited to only having boats for patrolling coastal waters. The Type VII’s were basically designed to putt around the Baltic and sending them on long patrols to the North Atlantic was really pushing them. The bigger Type-IX’s (like U-505) were more comparable to to the US Fleet Boats although the latter definitely had more creature comforts.
Not on the most luxurious boats in the Silent Service, no - or at least, not as extensively as was common in other navies where pretty much only the officers had their own beds, if that. Since they were built for long deployments across the Pacific and because they were larger and roomier than other subs (as needed for larger fuel and torp supplies), the fleet boat designers made sure the crew was relatively comfy. They figured that a tired, grumpy and lice-ridden crew loses efficiency.
Which is not to say all bunks were Holiday Inn quality - there were cots tucked between torpedo racks for example.
USS Razorback had a bunk over a nuke. I guess it didn’t matter how close one was to it, though. 
One surmises that when the balloon went up, you fired it out a rear tube at the fleet you’d been shadowing and ran like hell.
Before the fleet boats were in the fleet, they were called sewer pipes.