outlet steam temperature of a steam coil

[simplemath]

When design a steam coil, say steam into coil at 250psi and 400degF, what is the parameter when steam is out of coil?

This is a coil for heating water tank which is maintained at 35degF(ambient -40degF).

The outlet steam status decides required steam flow rate. I did the calculation and the results showed big difference if assuming

1. Phase change only. All steam to water, wall temperature constant.
2. Wet steam (50% of steam to water)
3. Subcool water. All steam to water, wall temperature drop below boiling point.

I would like to know what is the general practice and the reasoning behind it. Thanks.

--It boils down to simple math--

 

[gr2vessels]

Most of the energy of the steam is in the latent heat of condensation, so if you want the most efficient system, condense the steam in the coil with as little as possible subcooling from saturation temperature. Any 2 phase will increase the cost of the system without any benefit. Indeed, it boils down to simple maths.

 

[TBP]

You'll have a temperature control valve on the steam supply, and this will give you variable conditions downstream, depending upon the load. Once you've determined the maximum steam load, you can size the control valve, upstream & downstream piping, select the steam trap, etc.


Find your local representative for Spirax Sarco, Armstrong, Spence, Hoffman, etc. They're normally more than happy to look at your application, and walk you through it.

 

[HeviiGuy]

Harking back to years ago, in an earlier life:

If you use a temperature control valve, the process might at some point be satisfied. Then, the control valve will throttle down and cease admission of steam until it's required again. If the condesnate isn't allowed to drain by gravity, the coil may become waterlogged. When the valve opens, water hammer with potential coil damage may occur. Thus, if at any time there may not be enough steam pressure within a system to overcome head pressure, it's always important to allow free condensate drainage to a low point where it is then pumped to a higher elevation.
One more thing: A vacuum breaker (and thus also an air vent) must be installed in the system to not only allow free drainage but to also prevent any ingress of process fluid into the coil if there is leakage.

Ciao,

HevïGuy

http://www.heviitech.com

 

[simplemath]

Thanks all for your input.

In reality, what is the steam parameter coming out of steam coil?

Subcool Water? Water? wet steam?

I just how to know how much heat can be transferred to pipe wall from steam.

Thanks

--It boils down to simple math--

 

[HeviiGuy]

Short answer: It depends ;-)

Depends on what you're trying to heat
Depends on the volume that you're heating
Depends on the temperature rise
Depends on the heat transfer surface area
Depends on the coil material
Depends on the steam pressure

... and probably a few more that I've forgotten (it's been awhile)

What you really want is to have steam in the coil, trapped by a steam trap at its outlet. The latter provides the interface between the steam and condensate. Without it, you'll be wasting steam.

The aforementioned steam systems suppliers (Spirax-Sarco et al) really are an excellent resource. Make sure that you contact them directly rather than merely their local resellers.

Ciao,

HevïGuy

http://www.heviitech.com

 

[BillBirch]

Since it is a heating application,you should ensure that it is saturated steam entering the coil, i.e. remove any superheat, which is inefficient in terms of heat transfer (as gr2vessels suggests). Then, as HeviiGuy suggests, install a steam trap to separate off the condensate.
With the above configuration, the contents of the coil will be condensing saturaled steam. The ammount of steam required can be calculated from a simple enthalpy balance beterrn the steam (latent heat) the heating water.

 

[TBP]

As long as the steam supply to the control valve is saturated, the little bit of superheat you get downstream isn't a problem - it disappears quickly. If, on the other hand, you supply superheated steam to the valve, then you'll have LOADS of superheat downstream, and that WILL cause problems. Superheat is sensible heat, and you have to get past that to get to the latent heat, which is where all of the "action" is. Any significant amount of superheat in a steam heating application will make the heat exchanger behave as if it is air bound.

 

[Joesteam]

Considering all of the above, if you have a control valve on the incoming steam sensing the liquid temperature in the tank, and a steam trap on the outlet that has atmospheric discharge, then the condensate that leaves the steam trap (no steam will leave) will be at or near the saturation temperature, it will then flash down to 212oF and a portion of the liquid condensate will turn back into atmospheric steam.

Joe Lambert

http://www.control-associates.com/