Thermal Modeling of the Cooling Ceiling Systems as Commissioning Tool

This paper presents the results of a study performed to develop a computational model of cooling ceiling systems. This one has to be used in situ, as diagnosis tool in commissioning process in order to determine the main operating conditions of the system in cooling mode. The model considers the cooling ceiling as a fin. Only the dry regime is considered. From ceiling and room dimensions, material description of the cooling ceiling and measurement of supply water mass flow rate and air and water temperatures, the model calculates the cooling ceiling capacity, ceiling surface average temperature and water exhaust temperature. Fin efficiency, mixed convection close to the cooling ceiling (generated by the ventilation system) and panel perforations influence are studied. A series of experimental results (Laboratory test conditions) got on four types of cooling ceilings are used in order to validate the model. NOMENCLATURE A Area, [m] C Factor, [-] c Specific heat, [J kg -1 K] D Diameter, [m] h Superficial (convection and/or radiation) heat transfer coefficient, [W m K] k Thermal conductivity, [W m K] L Length, [m] M Mass flow rate, [kg s] N Number NTU Number of transfer units, [-] P Pressure or Perimeter, [Pa] or [m] Q Heat flow, [W] Q l, Heat flow per unit length, [W m] q Heat flow density, [W m] R Thermal resistance per unit length,[m K W] t Temperature, [°C] U Overall heat transfer coefficient, [W m K] w Distance between tubes, [m] W Width Greek symbols ε Effectiveness, [-] δ Thickness, [m] ρ Density or Ceiling panel porosity factor, [kg m] or [-] ∆ T Temperature difference, [K] μ Dynamic viscosity, [Pa. s] Subscripts a Air b Distant between tube axis and ceiling surface c Characteristic or cross-sectional cc Cooling ceiling conv Convective e External ex Exhaust exp Experimental f Fictitious , façade or fin h Convection i Internal meas Measured mr Mean radiant p Panel or panels blocks connected in parallel rad Radiative res Resultant su Supply s Panels connected in series or surface sim Simulated t Tube w Water x Fin distance 0 Fin base INTRODUCTION Thermally active cooling ceilings have been in successful use for many years in commercial applications, with a high percentage of sensible heat removed and low energy consumption. According to Conroy et al. (2005), cooling ceiling systems significantly reduce the amount of air transported through the building (often only about 20% of the normal all-air system air flow rates. This results in the reduction of the fan size, energy consumption and ductwork cross-sectional dimensions (Feustel and Stetiu 1995). Considering the large surface available for heat exchange, the water temperature is only slightly lower than the room temperature; this small difference allows the use of either heat pump with Eleventh International IBPSA Conference Glasgow, Scotland July 27-30, 2009