.BuildingSystems.Fluid.HeatExchangers.BaseClasses.WetCoilWetRegime

Information

This model implements the calculation for a 100% wet coil.

The equations from Braun (1988) and Mitchell and Braun (2012a and b), which are essentially the extension of the ε-NTU approach to simultaneous sensible and latent heat transfer, are utilized.

The mathematical equations are analogous to that of the sensible heat exchanger. However, the key distinction is that the heat transfer is driven by an enthalpy difference not by an temperature difference. This change in the driving potential results in re-defining capacitances and heat transfer coefficients accordingly.

The total heat transfer rate is expressed as

Q_tot=ε* C*_min (h_air,in-h_sat(T_wat,in)),

where ε*=f(Cr*,NTU*) and f is the same ε-NTU relationships (depending on the heat exchanger configuration) for the sensible heat exchanger.

h_air,in and h_sat(T_wat,in) are the specific enthalpies of the incoming moist air and saturated moist air at the water inlet temperature.

The capacitances of water and air streams are defined as

C*_air=m_air and C*_wat=m_watc_p,wat/csat,

where csat is an specific heat capacity, which indicates the sensitivity of the enthalpy of the staturated moist air w.r.t. the temperature, and is defined here as csat=(h_sat(T_wat,out)-h_sat(T_wat,in)) /(T_wat,out-T_wat,in).

The capacitance ratio and minimum capacitance are naturally defined as

Cr*=min(C*_air,C*_wat)/max(C*_air,C*_wat) and C*_min=min(C*_air,C*_wat).

The number of transfer unit for the wet-coil is defined as NTU*=UA*/C*_min, where

UA*=1/(1/(UA_air/c_p,air)+1/(UA_wat/csat).

References

Braun, James E. 1988. "Methodologies for the Design and Control of Central Cooling Plants". PhD Thesis. University of Wisconsin - Madison. Available online.

Mitchell, John W., and James E. Braun. 2012a. Principles of heating, ventilation, and air conditioning in buildings. Hoboken, N.J.: Wiley.

Mitchell, John W., and James E. Braun. 2012b. "Supplementary Material Chapter 2: Heat Exchangers for Cooling Applications". Excerpt from Principles of heating, ventilation, and air conditioning in buildings. Hoboken, N.J.: Wiley. Available online.

Revisions

• April 18, 2023, by Michael Wetter:
  Set start value for NTUAirSta.
  This is for IBPSA, #1728.
• Jan 21, 2021, by Donghun Kim:
  First implementation.