Nomenclature check for value reported on HX spec sheet

[jari001]

Hi everyone,

I received a spec sheet for a HX on site from back in the early 90s. On the spec sheet was the following item: MTD (CORRECTED) °F 18.57

Is the wording MTD (CORRECTED) the same thing as log mean temperature delta (LMTD)? I have never seen anything but LMTD quoted or discussed before. The value given is reasonable compared to the LMTDs I calculated for the HX but I want to make sure.

Thanks!
~J

 

[speco]

Since very few heat exchangers are actually in pure counterflow, there is usually some kind of correction factor (less than 1.0) that needs to be applied to the raw LMTD, which is just calculated from the two terminal temperature differences. That is likely what they are referring to on the data sheet. Put another way, it is corrected to adjust for being counter-crossflow, or maybe all crossflow.

 

[georgeverghese]

There are 2 correction factors to the LMTD

a) The temperature approach correction factor, commonly written as F. See TEMA design books, or Perry or DQ Kern for computation of this factor - this depends on the no of shellside and tubeside passes per HX.

b) A factor which takes into account the non linearity of the the heat release with temp over the entire length of the HX. This gets more complicated in multicomponent reboilers and condensors. In single phase units where Cp varies little with temp, this factor tends to 1.0

 

[speco]

George's answer is a bit more complete than mine. For cases where the heat released or absorbed is non-linear, other correction factors apply, and need to be considered. Typically, these types of cases need to be analyzed in different zones to account for the other effects. That is, for example, desuperheating, condensing, and subcooling zones in a condenser.

 

[jari001]

I will definitely check my copy of Perry's for the formula for F. I'll pull the cut sheets to check if there is anything extra in this HX.

Thanks!
~J

 

[Christine74]

Modern software programs like HTRI/HTFS do not calculate the LMTD, that's normally only done for hand calculations. Real heat exchangers typically don't have logarithmic temperature profiles.


-Christine

 

[jari001]

@georgeverghese
Perry's has the correction factor table, so now I have to collect some more data to calculate the plotted ratios.

@Christine74
That's interesting, haven't run across such a scenario yet. The particular HX I am working with was built in 1990, would that fall within the time period you are considering under the label "modern"? The HXs I've had to dig up info for range in built years of late 80s to late 90s and the spec sheets always had some version of a mean temperature difference value. Maybe if I ever have a chance to specify or work on equipment that isn't older than me I will see this

Maybe LMTD was used because it's familiar to all engineers that studied heat transfer without having to be experts but now its so easy to have software give you graphs and 3D models that the simplification isn't needed to convey the information. But I digress

 

[georgeverghese]

One thing to remember is that for the thermal design case, a thumb rule thermal designers follow is not to go below an Ft value (temperature approach component of the LMTD corr factor) of 0.8 for new designs and definitely not less than 0.75 for revamps.