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Abstract
Energy recovery ventilators are an effective way to reduce energy consumption in buildings while satisfying ventilation standards. As membrane technology and manufacturing methods improve, membrane-based energy recovery ventilators are seeing increased market penetration. In the present study, the feasibility of membrane energy recovery ventilators designed to fit in a commercial building wall cavity is investigated. A heat and mass transfer resistance and pressure drop model is developed for a low aspect ratio (width/height) exchanger and is used to evaluate the sensible and latent effectiveness of a counter-flow energy recovery ventilator with internal support structures. The performance of strip-fin and pin-fin structures are compared and dominant heat and mass transfer resistances are investigated. It is shown that for all cases the sensible heat transfer is dominated by the convective resistance while the dominant mass transfer resistance shifted to the membrane at smaller hydraulic diameters. The results suggest that as membrane technology improves, enhancements to the airside heat and mass transfer coefficients will be required to continue to realize performance gains.
Nomenclature
| A | = | area (m2) |
| AR | = | aspect ratio ( − ) |
| C | = | concentration (mol · m− 3) |
| cp | = | heat capacitance (J · kg− 1 · K− 1) |
| d | = | pin diameter (m) |
| D | = | diffusivity (m2 · s− 1) |
| DH | = | hydraulic diameter (m) |
| f | = | friction factor ( − ) |
| F | = | header correction factor ( − ) |
| h | = | heat transfer coefficient (W · m− 2 · K− 1) |
| hch | = | channel height (m) |
| i | = | enthalpy (J · kg− 1) |
| j | = | Colburn j factor ( − ) |
| k | = | thermal conductivity (W · m− 1 · K− 1) |
| K | = | overall transfer coefficient (m · s− 1) |
| l | = | segment length (m) |
| Lp | = | pin length (m) |
| m | = | mass (kg) |
| MW | = | molecular weight (kg · mol− 1) |
| n | = | number of moles ( − ) |
| N | = | quantity ( − ) |
| Nu | = | Nussult number ( − ) |
| P | = | pressure (Pa) |
| = | perimeter (m) | |
| Pr | = | Prandtl number ( − ) |
| Q | = | heat transfered (J) |
| R | = | resistance (K · W− 1, s · m− 3) |
| Re | = | Reynolds number ( − ) |
| Sp | = | streamwise center-to − center pin spacing (m) |
| St | = | Stanton number ( − ) |
| T | = | temperature (K) |
| Tp | = | transverse center-to − center pin spacing (m) |
| U | = | overall transfer coefficient (W · m− 2 · K− 1) |
| V | = | velocity (m · s− 1) |
| W | = | width (m) |
| Greek symbols | ||
| α | = | thermal diffusivity (m2 · s− 1) |
| β | = | convective mass transfer coefficient (m · s− 1) |
| δ | = | membrane thickness (m) |
| ϵ | = | effectiveness (%) |
| θ | = | ERV header angle (°) |
| ρ | = | density (kg · m− 3) |
| φ | = | relative humidity (%) |
| ω | = | absolute humidity ( − ) |
| Subscripts and superscripts | ||
| a | = | dry air |
| c | = | cross section |
| ch | = | channel |
| d | = | duct |
| e | = | exhaust air stream |
| f | = | fresh air stream |
| fg | = | vaporization |
| HT | = | heat transfer |
| i | = | inlet |
| l | = | latent |
| LM | = | log-mean |
| m | = | membrane |
| MT | = | mass transfer |
| o | = | outlet |
| p | = | pin |
| s | = | sensible |
| sat | = | saturation |
| t | = | transverse direction |
| v | = | vapor |
| w | = | wall |