Abstract
A study is described regarding the influence of the altitude, from 0 to 4,217 m (corresponding to atmospheric pressure from 101,325 to 60,000 Pa), on the behavior of a simple solid desiccant system used for air dehumidification purposes. The heating coil and the desiccant wheel are the main components investigated. The effectiveness method is used to evaluate the global behavior of the heating coil, and a detailed numerical model developed by the authors is used to predict the behavior of the desiccant wheel. Fixed-mass and fixed-volume airflow rate operations are considered in the comparison of the results at different altitudes. Two modes of specifying the inlet states of both airflows in the system are taken into account: (1) temperature and water vapor content and (2) temperature and relative humidity. As the atmospheric pressure decreases, the heat and mass transfer rates increase or decrease, depending on the mode of fixing the airflow rates and the inlet states of both airflows. Correction factors are determined for fixed-volume and fixed-mass airflow rate operations. The results show that these correction factors are also affected by the rotation speed of the desiccant wheel. Sea level data can be adopted for sizing the system without the need of correction when fixed-mass airflow rate and specifying the inlet states by the temperature and water vapor content.
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Abbreviations
- \( a \) :
-
Coefficient in Eq. (15)
- \( {A} \) :
-
Transfer area (m2)
- \( b \) :
-
Exponent in Eq. (15)
- \( c \) :
-
Exponent in Eq. (15)
- \( c_{p} \) :
-
Specific heat (J kg−1 °C−1)
- \( c_{\text{pf}}^{ *} \) :
-
Specific heat of the moist air (J kg−1 ºC−1)
- \( {C} \) :
-
Heat capacity rate (J s−1 ºC−1)
- \( {\text{CR}} \) :
-
Ratio of heat capacity rates
- \( d_{h} \) :
-
Dydraulic diameter (m)
- \( D_{f} \) :
-
Diffusion coefficient of water vapor in the moist air (m2 s−1)
- \( F \) :
-
Correction factor for the actual exchanger configuration
- \( F1,\,F2 \) :
-
Combined potentials or characteristic potentials
- \( h \) :
-
Specific enthalpy of moist air (J kg−3)
- \( h_{h} \) :
-
Convective heat transfer coefficient (W m−2 ºC−1)
- \( h_{m} \) :
-
Convective mass transfer coefficient (m s−1)
- \( {H} \) :
-
Altitude (m)
- \( j_{h} \) :
-
Heat convective flux (W m−2)
- \( j_{m} \) :
-
Mass convective flux (W m−2)
- \( \dot{J}_{h} \) :
-
Heat transfer rate per unit of transfer area of a desiccant wheel (W m−2)
- \( \dot{J}_{m} \) :
-
Mass transfer rate per unit of transfer area of a desiccant wheel (kg s−1 m−2)
- \( k \) :
-
Correction factor for the effect of pressure decrease on the heat or mass transfer rates
- \( {\text{Le}} \) :
-
Lewis number
- \( m \) :
-
Mass (kg)
- \( \dot{m} \) :
-
Mass flow rate (kg s−1)
- \( n_{\phi } \) :
-
Exponent in Eq. (18)
- \( {\text{NTU}} \) :
-
Number of transfer units
- \( {\text{Nu}} \) :
-
Nussel number
- \( P \) :
-
Atmospheric pressure (Pa)
- \( { \Pr } \) :
-
Prandtl number
- \( {P_{v}} \) :
-
Partial pressure of water vapor (Pa)
- \( P{}_{\text{vs}} \) :
-
Saturation pressure of water vapor (Pa)
- \( \dot{Q} \) :
-
Heat transfer rate (W)
- \( R \) :
-
Gas constant (J kg−1 ºC−1)
- \( {R} \) :
-
Thermal resistance (ºC W−1)
- \( {\text{Re}} \) :
-
Reynolds number
- \( {\text{Sh}} \) :
-
Sherwood number
- \( T \) :
-
Temperature (ºC)
- \( T_{\text{sat}} \) :
-
Water saturation temperature (ºC)
- \( {U} \) :
-
Overall heat transfer coefficient (W m−2 ºC−1)
- \( w_{v} \) :
-
Water vapor content (kg kg−1)
- \( v \) :
-
Specific volume of moist air referred to the unity of dry air mass (m3 kg−1)
- \( v^{*} \) :
-
Specific volume of the moist air (m3 kg−1)
- \( V \) :
-
Volume (m3)
- \( \dot{V} \) :
-
Volume flow rate (m3 s−1)
- \( X_{\ell } \) :
-
Adsorbed water content in the desiccant (kg kg−1)
- \( \Updelta T_{\text{lm}} \) :
-
Log mean temperature difference for a heat exchanger (ºC)
- \( \varepsilon \) :
-
Effectiveness
- \( \varphi_{v} \) :
-
Water vapor mass fraction
- \( \eta_{F1} ,\,\eta_{F2} \) :
-
Effectiveness parameters based on variations in F1 and F2, respectively
- \( \eta_{\psi } ,\;\eta_{h} \) :
-
Effectiveness parameters based on variations in \( {\psi } \) and \( h \), respectively
- \( \lambda \) :
-
Thermal conductivity (W m−1 ºC−1)
- \( \psi \) :
-
Ratio of the partial vapor pressure to the saturation vapor pressure
- \( \psi^{*} \) :
-
Relative humidity
- \( \rho \) :
-
Density (kg m−3)
- \( \sigma_{w} \) :
-
Density ratio when \( {T} \) and \( {w}_{v} \) remain constant
- \( \sigma_{\psi } \) :
-
Density ratio when \( {T} \) and \( {\psi } \) remain constant
- \( a \) :
-
Air
- \( {\text{cf}} \) :
-
Counter-flow heat exchanger
- \( {\text{coil}} \) :
-
Heating coil
- \( f \) :
-
Airflow
- \( {\text{in}} \) :
-
Inlet
- \( { \hbox{min} } \) :
-
Minimum value
- \( { \hbox{max} } \) :
-
Maximum value
- \( {\text{out}} \) :
-
Outlet
- \( {\text{ref}} \) :
-
Reference condition at sea level
- \( s \) :
-
Surface
- \( {\text{st}} \) :
-
Standard air condition
- \( v \) :
-
Water vapor
- \( w \) :
-
Water
- \( \phi \) :
-
Air (a) or water (w)
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Ruivo, C.R., Costa, J.J., Figueiredo, A.R. (2014). Influence of Altitude on the Behavior of Solid Desiccant Dehumidification System. In: Nóbrega, C., Brum, N. (eds) Desiccant-Assisted Cooling. Springer, London. https://doi.org/10.1007/978-1-4471-5565-2_4
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DOI: https://doi.org/10.1007/978-1-4471-5565-2_4
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