The method of the invention is based on the comparison of local vapor concentrations at the inlet and outlet refrigeration ports and taking action based on that comparison. Control apparatus incorporating the invention is installed inside a refrigeration enclosure, adjacent to a port, preferably at the lowermost port. If the enclosure contains multiple ports at similar height, then each port has a form of the control apparatus attached to it. The control apparatus adjusts a flow of vapor leaving the interior of the enclosure. The control apparatus includes a duct assembly and a blower system. The bottom portion of the duct assembly is a tunnel enclosure through which a conveyor belt passes. Connected to an inside edge of the tunnel enclosure is a duct that extends upward from the conveyor belt. A blower for this duct either sucks vapor away from the conveyor belt or blows vapor from the enclosure interior toward the belt. Regardless of the flow direction, a vapor curtain forms inside the tunnel enclosure and represents a transitional region from all vapor to all air. Control of the blower for the duct assembly is based on vapor concentrations in the tunnel enclosures adjacent to each port. A microprocessor compares measured concentration levels and alters the blower motor frequency in such a manner as to minimize the difference in concentration levels at each port.