Comparing STEAM energy and energy loss

[ddkm]

How do we compare the energy levels between steam at two different pressures? Assumption is saturated steam.

Example:
If I have two separate steam supply lines, one at 30 bar, and the other at 50 bar, both at the same flowrates and same pipe sizing. Both are NON-insulated, and assuming I only have money to install insulation for one of those lines, which one do I choose to maximize my value-for-money? (i.e. which energy loss is higher)


Answer/Solution:
Obviously, the answer is obvious, but how do I show and prove this calculation?

(a) Method A - Using "Enthalpy" values:
If I purely use Enthalpy values for the vapor phase:

H for steam @ 30 bar = 2802 kJ/kg

H for steam @ 50 bar = 2794 kJ/kg

then we find that enthalpy of the higher pressure/temp phase is actually slightly lower!

(b) Method B - Using Heat Transfer calculation:
Obviously also, the more effective way to show losses would be to do a Heat Transfer calculation whereby the higher pressure steam (therefore, higher temperature) will have a higher Delta T across the atmosphere, which will result in higher heat transfer (and therefore heat losses).

My quetion is, why doesn't the "easy" way of using Enthalpy values make sense??




Regards,
DD

---engineering your life---

 

[25362]

Have you considered the costs involved in producing each steam grade ?

 

[PropulsionAccess]

If you are working with water at a saturated vapor state, the temperature of the 30 bar (3000 kPa) will be around 234C, and the 50 bar (50000kPa) will be around 264C. This confirms your delta T as being bigger for the 2nd way. As for the enthalpy, if you had the exit conditions for each one, you could use the steady state, single flow energy equation: q = h(totalexit)-h(totalinlet)+work. Then you could use the enthalpies to show which condition had the highest q.

-carp

http://www.PropulsionAccess.com/

Propulsion Engineering Community

 

[quark]

That is a good question. I will only attempt to answer it.

First thing is that your assumption of saturated steam in both cases is valid if you operate two different boilers at 50barg and 30 barg. Otherwise, if you have even 1% moisture by running a 50barg boiler at 30barg, the enthalpy value reduces. If you are reducing the steam pressure from 50barg to 30barg by a pressure reducing valve, heat is lost in terms of superheat (unless you desuperheat it). Overall enthalpy remains same if you desuperheat the steam.

Lower heat loss increases wetness in the 50barg steam (when compared with 30barg steam) so your protection should be towards lower enthalpy system.

Most importantly, like 25362 suggested, you shouldn't forget the pumping cost of feed water (it is about 60% higher)

What bothers me is the pipe sizing. For the same mass flowrate, specific volume of 50barg 0.0386cu.mtr/kg and that of 30barg is 0.0645cu.mtr/kg. You already have higher pressure drop.

 

[25362]

The intantaneous heat loss would be larger for the higher saturated steam temperature and pressure but the percentual loss from the carrying pipes would depend on the flow rates, a factor missing in ddkm's presentation.

It is for me evident we're speaking of a thought process which apparently bears little if any resemblance with real situations. No wonder ddkm selected for comparison the steam pressure (~30 bara) with the highest possible enthalpy for saturated steam.

 

[ddkm]

25362, spot on. Obviously, my example is bordering on fictitious, but I chose it so that we have a similar basis to compare. Bottom-line is, why is there an apparent "contradiction" in terms of the Enthalpy of the steam (vapor)?

For the rest of the replies, thanks for the quick responses. I'll digest through them before replying.



Regards,
DD

---engineering your life---

 

[25362]

Some more points:

1. Although the heat loss per unit area of the pipes' is greater for the hotter steam, it may happen that the 30-bara steam loses more heat if it flows in a larger-diameter pipe (because of friction drop considerations as mentioned by quark) due the greater area of exposed surface.

2. How would the exercise look like if the heat-loss comparison referred to 17 bara- and 50-bara saturated steam both having about the same enthalpy ? .

3. Finally not all Joules have the same unit price, it depends on the application, and the generation system used.

 

[davefitz]

The most valuable steam source can be determined afer you define what you are going to do with the steam. If you are going to do work ( ie run a steam turbine), then you woul need to calculate which steam source would do the most work. Usually this work is estiamted as being some fraction of the isentropic expansion of the steam from initial Pi + Ti ( or Hi, Si) to the final exhaust condition at Pe. The work done is some fraction ( perhaps 80% ) of teh change in enhtalpy.

If you are going to use this energy only for heating some process, then you first determine if the process has a tight "pinch" temperature or if theprocess is primarily needs only the saturation temperature. If tehprocess dows have atight "pinch", then you will need to insulate the higher temp steam stream.

 

[MAEC]

The exact question I had to handle 5 months ago or so. Two more things I had to consider besides what was already exposed by the other members:

1. Availability. It will depend on the final process and reference.

2. Critical insulation radii.

Check those and see what happens.

MAEC

 

[ifreeman]

ddkm,

I would like to use entropy instead of enthalpy.

Thanks to PropulsionAccess, saturated temperature for 30
bar will be around 234C(507K), and the 50 bar will be
around 264C(537K).

Assumed that same amount of heat is transferred from
steam system to atm, the entropy change will be,
*Amb. T = 30C(303K)

30 bar : 2802/303 - 2802/507 = 3.721kJ/C/kg
50 bar : 2794/303 - 2794/537 = 4.018kJ/C/kg
(same condensation rate -same kg of steam condensation- is assumed)

As shown above, the etropy increase for 50 bar is larger
than 50 bar case, which means we are losing more valuable heat in case of 50 bar.

 

[ifreeman]

sorry for typographic error in the above posting.
"than 50 bar case" shall be corrected to
"than 30 bar case"