![Sample our Built Environment journals, sign in here to start your access, Latest two full volumes FREE to you for 14 days]({"type":"image" , "src" : "/sda/5926/TOC-Subject-Sample-2021-Built-Environment.jpg"})
Abstract
Biological fouling (biofouling) on cooling coil surfaces acts as thermal insulation, impeding heat transfer from air to coil surfaces, decreasing airside heat transfer coefficient and degrading coil cooling capacity. It is also a common cause of low ΔT syndrome in chilled water distribution systems. The effects of a commercially available ultraviolet germicidal irradiation system installed in a variable air volume system on the airside heat transfer coefficient, cooling coil capacity, and its potential to mitigate low ΔT syndrome were investigated via a field test. Energy-related measurements including chilled water supply/return temperature, water-/airflow rate and entering/leaving air temperature/relative humidity commenced 4 months before turning on ultraviolet lamps and continued for 10 months after ultraviolet germicidal irradiation intervention. The effects of the ultraviolet germicidal irradiation system were evaluated via a “before ultraviolet” and “after ultraviolet” comparison. After ultraviolet intervention, within the face velocity range of 1.5–3.0 m/s, the airside heat transfer coefficient increased by 11.8%–20.1%, which translated into 8.8%–10.2% increase in the overall enthalpy-based thermal conductance. The coil total cooling capacity and latent cooling capacity increased by 3.3%–3.8% and 4.5%–5.7%, respectively. The chilled water flow rate required to maintain the leaving air temperature set-point decreased by 8.0%–11.9% and the water-side temperature difference increased by 0.4°C–0.6°C.
Nomenclature
| a | = | coefficient in Equation Equation35 |
| Af | = | fin surface area, m2 |
| Ai | = | water-side heat transfer area, m2 |
| Ao | = | airside heat transfer are, m2 |
| At | = | tube outside surface area, m2 |
| b | = | coefficient in Equation Equation35 |
| cp, a | = | specific heat capacity of air, kJ/(kg·K) |
| cp, sat | = | effective specific heat capacity of saturated air defined in Equation Equation9 |
| cp, w | = | specific heat capacity of water, kJ/(kg·K) |
| Di | = | inside tube diameter, m |
| ER | = | uncertainty of in the calculated variable |
| = | uncertainty in a measured variable | |
| fi | = | friction factor defined in Equation Equation20 |
| F | = | correction factor for flow arrangement |
| ha | = | airside heat transfer coefficient, W/(m2·K) |
| hfg | = | enthalpy of vaporization, kJ/kg |
| hm | = | airside mass transfer coefficient, kg/(m2·s) |
| hw | = | water-side heat transfer coefficient, W/(m2·K) |
| ia | = | enthalpy of air, kJ/kg |
| ia, in | = | enthalpy of entering air, kJ/kg |
| ia, in, dp | = | saturated air enthalpy at dew-point temperature of entering air, kJ/kg |
| ia, out | = | enthalpy of leaving air, kJ/kg |
| is | = | enthalpy of saturated moist air, kJ/kg |
| iw | = | saturated air enthalpy at chilled water temperature, kJ/kg |
| iw, in | = | saturated air enthalpy at chilled water supply temperature, kJ/kg |
| iw, out | = | saturated air enthalpy at chilled water return temperature, kJ/kg |
| kf | = | thermal conductivity of fin, W/(m·K) |
| kw | = | thermal conductivity of water, W/(m·K) |
| Le | = | Lewis number |
| LMED | = | log mean enthalpy difference defined in Equation Equation16 |
| ma | = | air mass flow rate, kg/s |
| mw | = | water mass flow rate, kg/s |
| M* | = | coefficient defined in Equation Equation31 |
| Mfb | = | coefficient defined in Equation Equation32 |
| NTUa | = | airside number of heat transfer units defined in Equation Equation25 |
| = | Prandtl number | |
| q | = | coil total heat transfer rate, kW |
| qa | = | airside heat transfer rate, kW |
| ql | = | latent heat transfer rate, kW |
| qs | = | sensible heat transfer rate, kW |
| qw | = | water-side heat transfer rate, kW |
| ri | = | equivalent circular fin inner radius, m |
| ro | = | equivalent circular fin outer radius, m |
| R | = | function of the measured variables |
| = | Reynolds number | |
| RH | = | relative humidity |
| Ta | = | air dry-bulb temperature, °C |
| T*a | = | coefficient defined in Equation Equation33 |
| Ta, in | = | dry-bulb temperature of entering air, °C |
| Ta, in, dp | = | dew-point temperature of entering air, °C |
| Ta, out | = | dry-bulb temperature of leaving air, °C |
| Tcondensation | = | condensation temperature, °C |
| Tf | = | fin surface temperature, °C |
| Tfb | = | fin base temperature, °C |
| Tft | = | fin tip temperature, °C |
| Ts | = | temperature of saturated moist air, °C |
| Tw | = | chilled water temperature, °C |
| Tw, in | = | chilled water supply temperature, °C |
| Tw, out | = | chilled water return temperature, °C |
| uw | = | water velocity, m/s |
| U | = | overall heat transfer coefficient for cooling and dehumidifying coils defined in Equations Equation11 |
| UA | = | overall enthalpy-based thermal conductance for cooling and dehumidifying coils defined in Equation Equation11 |
| Wa | = | air humidity ratio, kg/kg dry air |
| Wcondensation | = | saturated humidity ratio at condensation temperature, kg/kg dry air |
| Ws.f | = | saturated humidity ratio at fin surface temperature, kg/kg dry air |
| Ws, fb | = | saturated humidity ratio at fin base temperature, kg/kg dry air |
| xa | = | airside correction factor defined in Equation Equation12 |
| xw | = | water-side correction factor defined in Equation Equation13 |
| Xi | = | measured variable |
| α | = | coefficient in Equation Equation7 |
| β | = | coefficient defined in Equation Equation34 |
| ηf, wet | = | wet fin efficiency |
| ηo | = | overall fin efficiency defined in Equation Equation28 |
| λ | = | coefficient in Equation Equation7 |
| μw | = | dynamic viscosity of water, Pa·s |
| ρw | = | density of water, kg/m3 |
| δf | = | fin thickness, m |
Subscripts
| 0 | = | reference condition |
| 1 | = | operating condition |
| a | = | air |
| in | = | coil entering condition |
| out | = | coil leaving condition |
| w | = | water |
Funding
The financial support of the National University of Singapore in the form of research grant RP 296-000-132-112 is gratefully acknowledged.