# .Buildings.Fluid.HeatExchangers.WetCoilEffectivenessNTU

## Information

This model describes a cooling coil applicable for fully-dry, partially-wet, and fully-wet regimes. The model is developed for counter flow heat exchangers but is also applicable for the cross-flow configuration, although in the latter case it is recommended to have more than four tube rows (Elmahdy and Mitalas, 1977 and Braun, 1988). The model can also be used for a heat exchanger which acts as both heating coil (for some period of time) and cooling coil (for the others). However, it is not recommended to use this model for heating coil only or for cooling coil with no water condensation because for these situations, Buildings.Fluid.HeatExchangers.DryCoilEffectivenessNTU computes faster.

#### Main equations

The coil model consists of two-equation sets, one for the fully-dry mode and the other for the fully-wet mode. For the fully-dry mode, the ε-NTU approach (Elmahdy and Mitalas, 1977) is used. For the fully-wet mode, 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 equation sets are switched depending on the switching criteria described below that determines the right mode based on a coil surface temperature and dew-point temperature for the air at the inlet of the coil. The transition regime between the two modes, which represents the partially-wet and partially-dry coil, is approximated by employing a fuzzy modeling approach, so-called Takagi-Sugeno fuzzy modeling (Takagi and Sugeno, 1985), which provides a continuously differentiable model that can cover all fully-dry, partially-wet, and fully-wet regimes.

The switching rules are:

• R1: If the coil surface temperature at the air inlet is lower than the dew-point temperature of air at inlet, then the cooling coil surface is fully-wet.
• R2: If the coil surface temperature at the air outlet is higher than the dew-point temperature of air at inlet, then the cooling coil surface is fully-dry.
• R3: If any of the conditions in R1 or R2 is not satisfied, then the cooling coil surface is partially wet.

For more detailed descriptions of the fully-wet coil model and the fuzzy modeling approach, see Buildings.Fluid.HeatExchangers.BaseClasses.WetCoilWetRegime. and Buildings.Fluid.HeatExchangers.BaseClasses.WetCoilDryWetRegime.

#### Assumptions and limitations

This model contains the following assumptions and limitations:

Medium 2 must be air due to the use of various psychrometric functions.

When parameterizing this model with rated conditions (with the parameter `use_UA_nominal` set to `false`), those should correspond to a fully-dry or a fully-wet coil regime, because the model uncertainty yielded by partially-wet rated conditions has not been assessed yet.

The model uses steady-state physics. That is, no dynamics associated with water and coil materials are considered.

The Lewis number, which relates the mass transfer coefficient to the heat transfer coefficient, is assumed to be 1.

The model is not suitable for a cross-flow heat exchanger of which the number of passes is less than four.

By default, the flow regime, such as counter flow or parallel flow, is kept constant based on the parameter value `configuration`. If a flow reverses direction, it is not changed, e.g., a heat exchanger does not change from counter flow to parallel flow if one flow changes direction. To dynamically change the flow regime, set the constant `use_dynamicFlowRegime` to `true`. However, `use_dynamicFlowRegime=true` can cause slower simulation due to events.

#### Validation

Validation results can be found in Buildings.Fluid.HeatExchangers.Validation.WetCoilEffectivenessNTU.

#### 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.

Elmahdy, A.H. and Mitalas, G.P. 1977. "A Simple Model for Cooling and Dehumidifying Coils for Use In Calculating Energy Requirements for Buildings". ASHRAE Transactions. Vol.83. Part 2. pp. 103-117.

Takagi, T. and Sugeno, M., 1985. Fuzzy identification of systems and its applications to modeling and control.  IEEE transactions on systems, man, and cybernetics, (1), pp.116-132.

## Revisions

• February 3, 2023, by Jianjun Hu:
Added `noEvent()` in the assertion function to avoid Optimica to not converge.
This is for issue 1690.
• January 24, 2023, by Hongxiang Fu:
Set `flowRegime` to be equal to `flowRegime_nominal` by default. Added an assertion warning to inform the user about how to change this behaviour if the flow direction does need to change.
This is for issue 1682.
• March 3, 2022, by Michael Wetter:
Removed `massDynamics`.
This is for issue 1542.
• November 2, 2021, by Michael Wetter:
Corrected unit assignment during the model instantiation.
This is for issue 2710.
• Jan 21, 2021, by Donghun Kim:
First implementation.

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