(a) Field of the Invention
This invention relates generally to the commercial refrigeration art, and more particularly to improvements in product merchandisers especially designed for the low temperature refrigeration of frozen food products. 
(b) Description of Prior Art
Since about 1960 the commercial refrigeration industry has developed many food merchandisers having open front product display zones for the display and merchandising of frozen food products.  Examples of such prior art configurations utilizing ducted air flow and multiple air curtain control include the following patents:
______________________________________ U.S. Pat.  No.  Date Inventor ______________________________________ 2,794,325 June 4, 1957 Shearer 2,836,039 May 27, 1958 Weber 2,855,762 Oct. 14, 1958 Zehnder 2,862,369 Dec. 2, 1958 Simons 2,890,573 June 16, 1959 Lamb 2,936,596 May 17, 1960 Rainwater 2,952,992 Sept. 20, 1960 Voorhies 2,962,875 Dec. 6, 1960 Barroero 3,010,379 Nov. 28, 1961 Arzberger et al 3,063,252 Nov. 13, 1962 Lamb 3,063,253 Nov. 13, 1962 Dickson et al 3,063,254 Nov. 13, 1962 Dickson et al 3,063,255 Nov. 13, 1962 Fanick et al 3,094,851 June 25, 1963 Beckwith 3,122,892 Mar. 3, 1964 Beckwith 3,186,185 June 1, 1965 Bently et al 3,218,822 Nov. 23, 1965 Bently et al 3,287,929 Nov. 29, 1966 Beckwith 3,289,432 Dec. 6, 1966 Brennan et al 3,365,908 Jan. 30, 1968 MacMaster 3,369,375 Feb. 20, 1968 Gerweck et al 3,392,544 July 16, 1968 Perez 3,420,070 Jan. 7, 1969 Hermanson 3,517,526 June 30, 1970 MacMaster et al 3,850,003 Nov. 26, 1974 Beckwith et al 4,026,121 May 31, 1977 Aokage 4,144,720 Mar. 20, 1979 Subera et al 4,265,092 May 5, 1981 Abraham 4,302,946 Dec. 1, 1981 Ibrahim 4,314,453 Feb. 9, 1982 Abraham 4,648,247 Mar. 10, 1987 Takazawa et al 4,964,281 Oct. 23, 1990 Tanaka 5,048,303 Sept. 17, 1991 Campbell et al 5,138,843 Aug. 18, 1992 Tamayama et al ______________________________________
All frozen food merchandisers are designed with the primary objective of maintaining product temperatures in the display area at about 0.degree.  F. for frozen food and about -10.degree.  F. for ice cream, which in the past has required evaporator coil temperatures in the range of -10.degree.  F. down to -35.degree.  F. At lower coil temperatures, ice buildup on the evaporator coils is accelerated, and thus the frequency and/or duration time of coil defrosts has been higher with the result that defrost heat usually produces increases in product zone temperatures.  Furthermore, the inefficiency of prior art open front frozen food display cases has resulted in high energy consumption requirements.  Thus, the large energy costs coupled with the inherent problems of maintaining proper product temperatures for good quality shelf life resulted in a marketing trend to closed, glass front reach-in merchandisers.