Freeze protection for rv water tank

[The MG Pony]

Hey can you guys confirm my calcs.

109L water tank surounded by R-5 insulation
Pump circulating 1,135L per hour
I want to raise the water temp from 1c to 5c

According to my calcs I need a 120w heater and at that wattage it will take 4 hours

is that right?

Q = M*C*Dt

109/(4*3600)*4.2*4 = 0.127 Kw/h

This is to protect the tank from freezing, so I supose even warming it to 2 degrees centigrade would be sufficient. and keeping power levels low is all so the goal!

Thanks for the help!

[amitsaxena]

Dear Your calculation is not OK as, the tank cap is 109L hence your heating rate will depend upon that only and your circulation rate is 1135L/Hr, means you hv only 5.76 min to heat the water hence the required wattage will be as

=109*4.2*4*Factor of safety/(5.76*60)

Say FOS as 1.1
then your requirement is 6.5Kw,
Also if you want upto 2 ~3 °C water the you can go for 2~3 Kw heater

Thanks
Amit

[The MG Pony]

the tank & water are static, the pump mearly recircs the water with in the tank.

I want to keep the wattage as low as pluasible as it will be running off a 12v battery bank.

Thanks for pointing that out.

[frank]

If you wish to raise 109L of pure water (density = 1kg/l) through 4K then
Q = 109 x 4.19 x 4 = 1826.84kW instantaneous.
To reduce the energy input you need to divide by the time required to pull the load down.

As with your calculation, if you divide by 4 hours (14400secs) then you get 0.127kW.
This figure will need to be adjusted by the heat loss/heat gain factor from the surrounding air and the tanks thermal properties. If the air is colder than the tank then the time required to chill the water or the energy input can be reduced. Conversely, if the ambient is higher than the tank, the energy input or the pull down time will need to be increased.
The circulation rate of the water is immaterial, as it appears to be a closed system,providing that the water loop is not gaining or losing energy while it is outside of the tank

127w/12v = 10.6Amps. Your battery charge won't last long

[james 100]

Is the re circ pump within the tank? I know it will only be a small amount but the pump will be giving off some heat...

[The MG Pony]

hehe with a battery bank of 400AH working (800AH total capacity) but every watt I can shave off helps, I think I'll run the freeze protection during the day while I get good sun on my pannels and the out side ambient is higher.

The tank will be insulated with R-5 or so insulation as well as it being in the main cab heat loss should be minimal pump should add about 5 to 10w worth of heat. I hope that after I heat the tanks during the mid day that I wont have to further heat them durring the night when I'll be running totaly off the battery bank

10.6 * 4 = 42.4AH for the antifreezing system, A three way fridge sucks up 100AH in a night! a vapour compression 12v fridge only uses 50AH a night, so seems the freeze prevention is out don by the others! The L.E.D lighting consums a wopping 0.360A ( 40w worth of light/200+ Lumens) & the dvd player system 4.25

Thank you all greatly, I'm glad to see I still managed to use the formula right!

Any you guys need help with the electrical design or lighting designs for the boat or RV just fire me an email and I'll do it on the main forums, from battery bank design to power converters

[Brian_UK]

How about a small heat pump using one of the Danfos BDF units?

[The MG Pony]

How about a small heat pump using one of the Danfos BDF units?

I was toying with using some of the rejected heat from the 12v fridge I have, but sadly can't afford one of then nifty danfoss rigs atm, I need to buy more solar cells and a 400w wind mill (That's the one thats going to hurt the most!).

I'm thinking of deisil glow plugs, 25w each and nice and compact!

[Moris87]

I think the water Tanks That Can’t Be Recycled can be use for the another purpose. But that water should be stored.

[Gary]

Have you considered using a propane heater?

[wenger230]

You are right sir the tank will be insulated with R-5 or so insulation as well as it being in the main cab heat loss should be minimal pump should add about 5 to 10w worth of heat. I am totally agree with you.

water tank