Galvanized or cold-rolled steel sheets are used according to customer specifications. In the case of cold-rolled steel sheets, they are degreased and treated with phosphoric acid before being coated with anti-rust paint and finishing paint, and then baked, so the exterior appearance is beautiful and the anti-corrosion effect is high. Flame-retardant glass wool insulation or eco-friendly polyester insulation is attached to the inside of the casing to prevent condensation and increase the insulation effect to prevent heat loss and improve sound absorption.