charge equation, hydrocarbon, commercial refrigeration appliance, bottle cooler
The use of hydrocarbon refrigerants has become common practice for small commercial appliances. The charge of hydrocarbon is limited by safety regulations. Therefore, refrigerant charge has become a critical design parameter for these appliances. To assist manufacturers in their early stage of appliance design (i.e. component selection) an easy to implement charge equation has been developed. The equation estimates the total appliance refrigerant charge based on the refrigerant, the operating conditions and the internal volume of the components of the cooling system. This paper presents the charge equation developed and discusses the derivation, assumptions and underlying calculations in detail. It is shown that the influence of refrigerant mass flux cannot be neglected. However, a simple approximation of the effect of mass flux is shown to be sufficient. The equation is validated based on the total refrigerant charge of 10 charge optimized glass door bottle coolers, varying in size from 50 to 1200 dm3, having a cooling capacity between 50 and 1500 W and using hydrocarbon as the refrigerant. The validation showed an agreement within 15% between the calculated and the actual refrigerant charge of the appliances.