Passthrough window service openings shall be limited to 216 square inches each. The service openings shall not be closer together than 18 inches. Each opening shall be provided with a solid or screened window, equipped with a self-closing device. Screening shall be at least 16 mesh per square inch. Passthrough windows of up to 432 square inches are approved if equipped with an air curtain device. The counter surface of the service openings shall be smooth and easily cleanable.