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## DETAILS MEANING OF PARAMETERS “GLOBAL EXCHANGE COEFFICIENT” and “DHML”

Modified on: Tue, Oct 18, 2016 at 12:39 PM

FIRST SITUATION:

In the cooling coils where
the air-fluid cools down, the exchanged capacity is said “SENSIBLE CAPACITY”
if the water contained in the mixture (air-moist) does not condense. This
situation occurs when the average temperature on the base of the fin is major
than the dew temperature of mixture. In this case you can use the equation:

Q = K x S x DTML*F

where:

Q (W) is the exchanged capacity

K (W/(m2K)) is the Global
Exchange Coefficient

S (m2) is total area (primary + secondary)

DTML (K) is the average logarithmic difference calculated
considering the countercurrent flow

F is the factor that consideres that in a thermal exchange
coil the perfect countercurrent flow is not realistic

SECOND SITUATION:

In the cooling coils where the air-fluid is condensed, the system releases “LATENT CAPACITY”
too. So the total exchanged capacity is the sum of “SENSIBLE CAPACITY” and “LATENT CAPACITY”. In this case you can use the equation:

Qtot=K1*S*DHML*F

where:

Qtot (W) is the total exchanged capacity

K1 (W kg/ (m2 kJ)) is the Global Exchange
Coefficient referred to the enthalpic difference

S (m2) is total area (primary + secondary)

DHML (kJ/kg) is the average logarithmic enthalpy difference

F is the factor that consideres that in a thermal exchange coil the perfect countercurrent flow is not realistic

CONCLUSION:

If the total exchanged capacity is only SENSIBLE, the application
will show the K factor otherwise if there is the latent capacity too, the
application will show the K1 using the right units of measurement.

APPLICATION:

In the application it is possibile to inhibit this (right) behavior
by menu (“Tools” -> “Options” -> “Disable calculation with DHML”):

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