Ambient Temperature for Heat Loss Calculation

Actually, ASHRAE also has some of the 98%, 99%, and 99.4% data for a lot of international sites also. Given a location, one could probably find a listed location and make a reasonable approximation from the ASHRAE published tables.

If the tank is at ambient then there will be no heat loss, as heat only travels where there is a temperature gradient.

very good zen master. It sounds like it was a trick exam question.

Thermal energy only travels in one direction. Hot to Cold.

....unless one supplies a driving force in the form of a ground-source heat pump, a refrigerator, Stirling Engine running backwards, whatever....

Good answer.

Questions: what is in the tank, and what condition (temperature) does it need to be maintained at? If the temperature become "extreme", will the tank freeze, condense, or boil? Should anyone care whether the material in the tank freezes, condenses, or boils?

We probably need a bit more definition to the original question.

The teacher didn't specify or provide those items.

There has to be some heat loss as outside air temperature changes. Even though the tank is said to be operating at "ambient", the bulk temperaure of the tank contents takes some time to react to outside temperature changes. If there was no heat transfer, the bulk temperature of the contents would not change regardless of "ambient" temperature. That might be possible if the tank were properly insulated, but we don't know that from the original question.

Granted, this is a transient condition, but the energy balance still requires it until a new equlibrium is reached.

Example: if the tank temperature is at 75 deg F during the day, but the air temperature cools down to 50 deg F at night, there will be some heat transfer from the tank to the outside air until equilibrium is reached. The air next to the tank skin would raise somewhat while the bulk temperature would decrease until the two are at the same temperature. The heat energy transferred to the air would be dissipated in the surrounding air until the heat energy is so spread out that bulk air temperature effects are negligible.

If this is a trick exam question, it's mis-leading unless you give a very qualified answer.

Climate extreme will not matter at all. As pointed out in post #2, even if temperature falls to 0 deg. C, that is now ambient and no heat loss (or gain) will occur.

A possible better formulation of the question would be, like this. I have a large tank with a thin watery solution in it, that we can consider plain water. Average winter day temperature is XX, the nighttime temp may dip below 0C for yy hours. If this weather keeps up, what is the minimum size of the uninsulated container to avoid freezing and destroying the tank? I may have left some out, like safety margin, etc.... But, with such data you can begin to do some heat transfer calculations.

Quite. That sort of stuff isn't in the original post. What on Earth is the forum to go on (rhetorical question)?

Your question "indicates" that you have SOMETHING in the tank that is above ambient temperature that you want to keep "warm".

I assume that you are trying to calculate what level of heater you will need to support this requirement.

Calculate heat loss from highest expected temperature in the tank to the record low temperature for your area- WITH AN ALLOWANCE FOR THE HIGHEST WIND RECORDED IN WINTER. If you do not know how to calculate for moving wind, use a safety factor of 30%.

Your product-sensing thermostat will actuate your heater as required whatever the ambient temperature is at any time.

You just want to be sure that your product temperature stays at its design condition- the SLIGHT increase in cost for a slightly larger heater than you MIGHT need is negligible compared to possible product loss or other problems if the temperature falls below design temperature.

Since you are worried about low ambient temp.,I presume the storage fluid is hot.If there is a min. temp. to be maintained, the insulation to be provided is depends on that temp.

Another point is how long is the duration of low ambient temp?Insulation thickness also depends on economics.You must find out the optimum insulation thickness.

Thanks

I appreciate your concern on the subject, as I apologize for not reviewing the latter comments, since the first two posts were right on target. ("The first cut is the deepest")

As stated later, and clarifying the issue: The Tank is in a vertical position resting on a slab, it is to be kept at 120°C, it has 3" of fiber glass as insulating material (k= 0.039 W/mK, assumed). The thermal circuit is considering only the conduction thermal resistance; no transient from wind, not even the convection from external fiber glass- air interfase ( I submit to your consideration the accuaracy of this later assumption).

Once the thermal circuit for the worst case scenario is established, with the minimum temperature of design provided by ASHRAE (thanks again to first 2 posts); The amount of maximum heat, needed to keep the temperature in the tank relative constant would be known (based on the total surface of the thank, losses through the base support, accesories, etc)

The tank is heated by a boiler system that burns diesel to heat oil that goes into a heat exchanger in the tank; with a ON/OFF control scheme. I am considering to add heat from solar collectors (to be added) this heat could be added between the output of the tank and the return input of the boiler. I am considering a PID control scheme in which the the flow could be varied with a VFD in the pump.

I appreciate your advice, comments, suggestions and insight as well as your time.

Regards

Rather than use ASHRAE for lowest temperature- call your local weather station and get the lowest recorded temperature- EVER.

You DEFINITELY need to consider impact of wind- even thermal gradient convection "breeze" or- as stated earlier- just add 30% to your calculated losses.

By the way- you will be losing heat to the slab as well, and i suspect that there is NO insulation between the tank and the slab- assume 50F temperature below the slab (although some heat loss will increase that temperature) for safety.

This is quite a bit different than your original post. You included a lot more information, some of it irrelevant and some left out. For example, you are using a thermal oil for process heating and while it's nice to know it's heated by burning diesel fuel, the surce of heating doesn't really matter. But the temperature of the thermal fluid does and you left that out, as well as ignoring the effect of wind.....and as pointed out, the effect of wind on heat transfer can be considerable depending on your geographical location.

I'm not sure what you mean by a PID control scheme, or why you want to put solar and a VFD on the pump in such a system. You are making it overly complicated unless you need extremely precise temperature control (and if I am reading your posts correctly, you don't need it).

I recently designed a very similar system for storage tanks located in Canada, and the process temperature of 160 deg C can easily be maintained with thermal oil at 205 deg C using only an on-off flow control circuit. I would suggest you forget using the extra schemes or you will find the control system overly complex and inefficient.

BTW, as a general comment I will say that a problem correctly and completely stated is already half solved and we can give much more useful commentary when we know what you really want and need to know.

Thanks for your reply. I am considering the heat loss through the slab, as you stated it is not insulated. I assume the thermal relationship would be Q= (Tin-Tout)/(R1+R2);

R1=k1A1/t1; R2=k2A2/t2

Where Tin : temperature inside the tank

Tout: minimum temperature of ground ie 50°F

k1 thermal conductivity of tank, k2: thermal conductivity of concrete; t1: thikness of tank; t2: thikness of slab; A1=A2: Bottom Surface of tank.

As with the control I will take your advice as well and will keep the on/off scheme. So I will take into account a 30% for the convection effect. I am also aware that more analysis is needed in the heat exchanger, my first post was just in order to know where to obtain the minimum temperature of design.

I appreciate your concern

Regards

I did post the former, since I believe that the bottom surface power has to be proportional with the surface area (proportional with D^2). However I found an article from a provider (Tycohtermal controls) that assumes that the power is proportional to D (heat losses are proportional to perimeter) see graph 6.2 page 23 http://goo.gl/RgGTC I am looking for losses for a diameter of 4.5mts (the graph in question does not include such a value so I need to make an interpolation or other technique of aproximation)

So I divided the power lost (Q) by D^2 and D (D=Diameter at desired ΔT) and as stated earlier found that the Q/D is relative more constat than Q/(D^2 ).

I appreciate your concern