I need some help on calculating the energy balance for a steam-water HX.  On the steam side, I am using the Q=m(hi-ho), hi=h(vapor) for steam inlet, ho=h(liquid) for condensate outlet, is that right or I should ho=h(liquid+vapor)? 

How do I determine whether the HX is efficient or not?  Comparing the desgin U value and calculated U value?

I am familar with Shell and tube heat exchangers so my response is written based on a shell and tube, but is probably valid for plate HE's as well.

When I see hi and ho in a heat exchanger calculation I think of inside and outside heat transfer coefficient of the tubes.  These units are W/m2.K (for example)/API Cast Steel Valves.  Q has units of J/s and m kg/s.  Therefore your equation is not dimensionally correct.

You post should state what you are assuming Q, hi, ho and m to be.  Please repost your question with sufficient detail.

FYI the heat transfer coefficient for the steam side when on the shell, is usually approximated as 6000 to 10000 W/m2.k.  In practice it may be higer than this.

I pressume murphymok refers to enthalpies (hi:inlet, ho:outlet) in which case his original formula would be OK, as long as these represent the true enthalpy of steam (wet or dry) and of the condensate if there is total condensation. Otherwise better work it out on the (non-evaporating) water side, where Q=mCp(To-Ti).

As for symbols, enthalpy is better indicated with a capital H, not to confuse it with heat transfer coefficients, generally denoted by a small h, as tickle says.

A capital U is generally used for overall heat transfer coefficients (OHTC).

There are different ways to review thte performance of a water heater. The ASME power test code PTC might be a good place to obtain typical methods.

One old fashioned method involved comparing the current TTD and DCA to the design TTD and DCA. The terminal temperature difference TTD is the difference between the shell saturation temperature and the outlet water temperature. If there is no desuperheater, then this number will be at its minimum at full load and increase to larger values as load drops.  

The drain cooler approach DCA is the difference in temperature between the liquid condensate drain and the inlet water temperature. It should trend the same way as TTD with load, and it primarily illustrates the effectiveness of the drain cooler.

You could do a more involved review by analyzing the  effectiveness of each  of the 3 zones- desuperheater, condensing zone, and drain cooler zone, using compact heat exchanger theory ( Kays ) . Each zone's performance  is represented by a zone effectivenes e, zone NTU ( number of transfer units) , ratio of specific heats R, number of passes, etc. This analsysi would permit you to determine which zone is not permforming properly.

A more basic performance impact is teh actual operating pressure of the shell. This can be lower than design due to part load operation of the steam system or higher than expected pressure drop in the steam supply lines.

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