Apparatus for chilling or freezing a product, comprising: an elongated enclosure having an entrance and an exit; a movable belt that can carry product to be chilled or frozen on the top surface of the belt within the enclosure between the entrance and the exit, the belt comprising an upper run and a return run below the upper run, and the enclosure including a first section which extends from the entrance in a generally downward sloping path, and a second section which extends in a generally upward sloping path to the exit, wherein the top surface of the belt lies on a path that crosses first and second points (31A, 31B) and that includes a segment (34, 36, 38) within the enclosure that is between the first and second points and that is lower than an imaginary horizontal line passing through the lower of the first and second points, and wherein the belt does not pass through any container of cryogenic liquid; wherein the enclosure includes first and second structures (40, 41) located within the enclosure between the first and second points (31A, 31B), each structure extending from above the imaginary line to below said line and above the top surface of the belt, the first and second structures impeding entry of air from the ambient atmosphere into the enclosure and, together with the top, bottom and sides of the enclosure, defining a chilling zone; the enclosure further including third and fourth structures (44, 45) located within the enclosure between the first and second points (31A, 31B), each structure extending from the bottom of the enclosure to below a return run of the belt, the third and fourth structures impeding entry of air from the ambient atmosphere into the enclosure and retaining cryogen vapor within the chilling zone; at least one outlet within the enclosure that is connected to a source of liquid cryogenic refrigerant and is capable of dispensing said cryogenic refrigerant into the chilling zone; and a vent (13, 14) at one or each of the entrance and the exit, located above said imaginary line, through which cold vapor from within the enclosure together with air from outside the enclosure can pass into an inlet of said vent, and further comprising a curtain extending toward said line from a location on said vent that is between the vent and the ambient atmosphere; wherein the height H of the entrance above the top surface of the belt is determined by the formula: H ≤ 1.5 * L * tan ⁢ β wherein: H is the height of the entrance above the top surface of the belt, β is the angle of the slope of the belt at the entrance, and L is the distance of the outermost of the third and fourth structures (44, 45). an elongated enclosure having an entrance and an exit; a movable belt that can carry product to be chilled or frozen on the top surface of the belt within the enclosure between the entrance and the exit, the belt comprising an upper run and a return run below the upper run, and the enclosure including a first section which extends from the entrance in a generally downward sloping path, and a second section which extends in a generally upward sloping path to the exit, wherein the top surface of the belt lies on a path that crosses first and second points (31A, 31B) and that includes a segment (34, 36, 38) within the enclosure that is between the first and second points and that is lower than an imaginary horizontal line passing through the lower of the first and second points, and wherein the belt does not pass through any container of cryogenic liquid; wherein the enclosure includes first and second structures (40, 41) located within the enclosure between the first and second points (31A, 31B), each structure extending from above the imaginary line to below said line and above the top surface of the belt, the first and second structures impeding entry of air from the ambient atmosphere into the enclosure and, together with the top, bottom and sides of the enclosure, defining a chilling zone; the enclosure further including third and fourth structures (44, 45) located within the enclosure between the first and second points (31A, 31B), each structure extending from the bottom of the enclosure to below a return run of the belt, the third and fourth structures impeding entry of air from the ambient atmosphere into the enclosure and retaining cryogen vapor within the chilling zone; at least one outlet within the enclosure that is connected to a source of liquid cryogenic refrigerant and is capable of dispensing said cryogenic refrigerant into the chilling zone; and a vent (13, 14) at one or each of the entrance and the exit, located above said imaginary line, through which cold vapor from within the enclosure together with air from outside the enclosure can pass into an inlet of said vent, and further comprising a curtain extending toward said line from a location on said vent that is between the vent and the ambient atmosphere; wherein the height H of the entrance above the top surface of the belt is determined by the formula: H ≤ 1.5 * L * tan ⁢ β wherein: H is the height of the entrance above the top surface of the belt, β is the angle of the slope of the belt at the entrance, and L is the distance of the outermost of the third and fourth structures (44, 45). H is the height of the entrance above the top surface of the belt, β is the angle of the slope of the belt at the entrance, and L is the distance of the outermost of the third and fourth structures (44, 45).