Size steam coil

[simplemath]

Need to size a water tank heating coil to prevent icing. I evaluated the heat load of the tank and get the heat transfer coefficient between pipe and water. One question is how to get the temperature difference: ambient is -40degF, water is to maintain 32-35degF and steam temperature after control valve is 400degF. Is it right just to assume steam temperature constant and use (400-35)degF as deltT ? The area calculate in this way is fairly small.

Another question is to get the outlet steam/condensate parameter. I use 2” pipe and has the steam property 250psi and 400degF after control valve. I do not know outlet steam parameters and can not calculate the steam flow rate required.

Please clarify me and if knowledge of code or book on this issue, please let me know.



--It boils down to simple math--

 

[ChrisConley]

Usually the energy in a steam system is the latent heat of vaporization that is given off when steam condenses (~1200 Btu/hr/lb)

I would calculate my heat loss from the tank at design conditions and then determine how much steam (lbs/hr) was needed to meet that heat loss.

A quick glance at a steam chart shows the temperature of saturated steam at 250 psi as 456F.

 

[TBP]

Chris - Check your steam tables. 250 PSIG is 406*F. 250 PSIA is about 401*F. I suspect simplemath is working in gauge pressure. Also, if you can get 1,000 BTU/lb out of steam, you'll be doing very well. The only way to get anything like 1,200 is to sub-cool the condensate to just above freezing. I've never encountered that, in the real world.

 

[simplemath]

Thanks ChrisConley and TBP for your valuable input.

TBP:
For general practice what is the amount of latent heat used for calculating the steam load? Is it reasonalbe to use 600BTU/lb to garentee the required load?

--It boils down to simple math--

 

[MintJulep]

This is a two-step calculation, and it seems you are trying to do it in a single step. The way to go about this is:

Step 1: Determine the heat loss from the tank to the surroundings. This will be made up of convection and radiation from the outer shell of the tank. The delta-T for these calculations will be 35-(-40). Don't forget that radiation calcs need to use absolute temperature. This will be the amount of heat that you need to add to the tank to prevent freezing.

Step 2: Calculate the steam flow rate needed supply this amount of heat. You know the steam inlet conditions, so look up the enthalpy. The steam outlet conditions are going to depend on your control devices and where the discharged steam is going. For example, if you have a metering valve and are controlling the flow of live steam through your heating coil and into a condensate system then your outlet condition is going to be saturated steam at whatever the pressure of the condensate system is. If you have a steam trap at the discharge then the outlet condition will be pretty close to saturated water at 210F.

 

[TBP]

I've always ballparked the BTU/lb as 1,000. It'll get you pretty close, and is easy to work with. If you can get a copy of the Spirax Sarco book "Hook-Ups", all will be revealed. It's excellent. If you're a steam newbie, try to get "Steam Utilization" - it's got a lot of good fundamental info in it.

Attached is a scan out of an old Crane catalogue, that may help you. A lot of people wrinkle their noses at the old stuff, but I've acquired a lot of old books, and the thing that strikes me, is not how much things have changed, but how little.

 

[ChrisConley]

Hi TBP,

His range of pressure is outside my usual range (I'm typically in the HVAC range of 12 psig), so I just grabbed the latent heat of vaporization of steam at 250 psig, which was listed at 1200 Btu/lb. The 456 was a slip of the keyboard

 

[RossABQ]

You haven't stated what the insulation is, but it better be good! Are there flows in/out of the tank? At what temperatures?

At these temperatures for the water, the steam enthalpy change will likely be down to subcooled conditions. Is this an external tracing coil, or an internal direct contact coil?

I'm not sure why you would want to maintain only 32 - 35 deg., but in (-40 F) ambient conditions you risk some of the water at the skin of the tank freezing. Any interuption of the steam flow risks freezing the tank. All this depends on the size of the tank, the relationship of the heating coil to the volume of water, etc.

 

[simplemath]

MintJulep: I calculated heat load just as step 1. Lacking of downstream information, I assume condensate piping same pressure as steam. I used 75% of inlet gauge pressure as steam pressure.Is it reasonable?

TBP:
The catalog is great. It provides a backup check for spiraxsarco numbers. By assuming the coil remains at saturated temperature, i will only consider latent heat of 840BTU/hr, sensible heat I assume is not as effective as latent heat.

RossABQ: Good question. Inflow has temperature higher than 35F. Just as you said and adding that no agitator, surface icing is also my concern. I could only account for this using a lower heat tranfer coefficient. Do you have any data on non-stirred water-steam application?

--It boils down to simple math--

 

[TBP]

Condensate piping pressure will have to be MUCH lower than the steam pressure. If they're the same pressure, the heating coil will simply water-log, since you won't have any pressure differential across the trap. Remember that steam traps are nothing more than automatic valves, that open when they sense air and/or water, and close when they sense steam.

If you install the heating coil near the bottom of the tank, the convection flow will likely provide enough circulation.

I once installed a steam heater as a retrofit in a fat tank. We bought an off-the-shelf shell & tube heat exchanger, removed the head and bundle. We cut the shell off a few inches past the flange. We cut a corresponding hole in the steel tank, and welded the flange section of the heat exchanger shell to it. We shoved the bundle through it, into the tank, and bolted the head back on. (We left enough of the shell on the flange to enable us to do-up the bolts to re-connect the head.) The tank was now, in effect, the shell of the heat exchanger. We ran the steam through the bundle, and it worked like a charm.