Heat Exchanger Calculation.

[murphymok]

Hi,

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?

Thanks in advance.

 

[tickle]

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). 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.

 

[25362]

I pressume murphymok refers to enthalpies (hi:inlet, houtlet) 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).

 

[siretb]

1) On the steam side Q=Hout-Hin; Hout is the total enthalpy, liquid + vapor if vapor fraction is not 0. (incomplete condensdation, some vapor left.

When you calculate or design the heta exchanger, be careful if the steam is supergeated. If it is you will have a dry area, where the heat transfer coefficients are much lower than in the condensing region.

2) The U , overall heat transfer coefficient will tell you if your heaa exchangeur is working properly (not fouled, ...), but is not a measure of effectiveness or efficiency, in a thermodynamical sense. It is a very valuable piece of information. Again, the overall U will be much different if you have a desuperheating region, of if you have not. The figures given by tickle seem high to me. I'd rather use U=1500-4000 W/m2/°K for shell & tube but I am not a heat exchanger expert. I'll appreciate some feed back here.

3) Performance / effectiveness You may calculate the P factor, unaccomplished temperature change=(t(cold stream out)-t(cold stream in))/(T(hot stream in)-t(cold stream in)),
or calculate the Effectiveness ratio E=heat duty exchanged/(H enthalpy hot stream in) - H(enthalpy that hot stream would have if it reached cold inlet stream temperature)). It compares the actual heat duty to the maximum enthalpy difference.

Hope this helps

 

[murphymok]

Hi guys,

I am analyzing a shell and tube HX and trying to compare the operating conditions with the design conditions to see whether it needs cleaning or not. I have the specs. for the design conditions. When in operation, on the steam side, I have the steam flow, steam inlet pressure and condensate outlet temperature. On the water side, I have temp in and out, but no flow. I was able to calculate the water flow based on: Qw=MwCp(to-ti)=Qs=Ms(hi-ho). I am thinking about using Q=UA(ti-to) to find out U and compare with design specs. Am I on the right path to find out the efficiency?

Million Thanks.

 

[davefitz]

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.

 

[25362]

1. Don't confuse efficiency that compares actual vs design (as new) performances, with effectiveness that relates actual to maximum possible heat transfer.

2. Q=UA(LMTD)_c, not simply UA ([Δ]t).

3. Yes, by estimating the value of U you can compare with the design value. But don't forget that changes in operating conditions, not only fouling, may modify results.

The value of U is dominated by the largest resistance. Since this is on the water side, then changing water flow rates or water source (hardness, particulates), and released air on heating, for example, may modify the value of U.

4. The heat transfer coefficient on the water side is about proportional to Pr^1/3.[κ]. Water's Pr changes from 9.4 at 10 deg C to 3.0 at 60 deg C, and the thermal conductivity [κ] (W/m.K) from 0.585 to 0.654, at those temperatures, respectively.
Resulting in a 31% change in the value of h.

h₁₀/h₆₀ [ε] (9.4/3)^1/3(0.585/0.654) = 1.31.

This means that just by changing water temperatures one may change the value of h_w and thus the value of U.

5. All said, even oil-free steam is debited with a fouling factor of 0.0001 m².K/W, from data taken from TEMA. Non-condensables, if expected to collect, should be duely removed.

 

[davefitz]

The LMTD approximation works well for the condensing zone, but for the desuperheater zone and drain cooler zone it is more accurate to use the e-NTU method by Kays, as it accounts for the limited number of passes etc.

 

[tickle]

Is the STHE vertical or horizontal?

How is the heat transfer controlled?

The reason I ask is because if the control is by a control valve on the condensate outlet, then the shell side (usual side for steam) will be partially flooded reducing the area for heat exchange

 

[murphymok]

Hi,

It's a horizontal STHE. Its operation is controlled by the demand of hot water usage in the building. I don't see any control valves on the HE. The data I can gather is the steam flow and temperature of both sides. I don't know the water flow rate. Any thoughts on calculating the efficiency of it?

Thanks.

 

[rmw]

murphymok,


Go to

http://www.processassociates.com,

and look for the tools available on this site.

If you use their tools to determine what they can produce, by the time you gather enough information to fill in all the fields they require, you will have your problem solved.

If you cannot glean by some means enough information to fill in all the blanks, (fields) then all we can do is guess along with you.

rmw

 

[murphymok]

Hi rmw,

I have gathered some data for the HE, if you or anyone can do a quick calculation, I can compare to mine

Steam In: 8 psi saturated @ 640 lb/hr
Steam Out: Condensate out at 210F through the trap
Water In: 60F city water
Water Out: 115F @ 90 psi

Thanks.