Surface Heat Transfer Coefficients for Steam/Air Mixtures in Two Pilot Scale Retorts M. A. TUNG, H. S. RAMASWAMY, T. SMITH, and R. STARK ABSTRACT Surface heat transfer coefficients (h) for steam/air mixtures were studied in a vertical, positive flow retort and a horizontal, forced circulation Lagarde retort, using rectangular metal bricks. Influences of test brick orientation and fractional steam content (S), tempera- ture, flow rate and direction of the heating medium on h values were studied. In both retorts, steamcontent was found to be the major factor (p < 0.05) influencing h, while temperature had no effect (p > 0.05). Flow direction and flow rate in the positive flow retort, and brick orientation in the Lagarde retort alsoinfluencedh. Exponential equations, h = a exp (bS), described relationships be- tween h and S, with a and b dependent upon retort type, flow rate, flow direction and test brick orientation. INTRODUCTION TWO HEATING MEDIA have been used to commercially sterilize foods in flexible pouches: water with air or steam overpressure and mixtures of steam and air. The common feature in each processing method is the necessity to main- tain a retort pressure greater than the pressure within the pouch during processing. This is necessary in the heating period in order to counteract the tendency of gaseswithin the pouch to expand and retard heat transfer, and during the cooling period to protect package integrity. While steam/air processes have been shown to be commercially feasible in Canada, Europe and Japan, there has been some hesitation to acceptance in the U.S.A. The reported lower surface heat transfer coefficient values for steam/air mix- tures as compared with water immersion/air overpressure heating (Pflug, 1964) appear to be of concern. Pflug and Borrero (1967) found that, if precautions were taken to assure constant flow of the heating media to avoid stagnant spots, steam/air processes could be effectively used. Adequate circulation of the heating media can be achieved either by a positive flow system or a powerful fan to create turbulent conditions within the retorts. Yamano and Komatsu (1969) and Yamano et al. (1969a, b; 1975) extensively studied the use of steam/air heating media. Their work provided a basis for the predominant use of this method in Japan, where modified conventional horizontal retorts are being used for steam/air processing (Lampi, 1979). Milleville (1980) reported that there were more than 300 Lagarde ‘air-over-steam’ Turbo Cookers in Europe; however, only a small number of these were used in retort pouch processing. A continuous horizontal retort, Rex- ham’s Hydrolok Sterilizer (Rexham Corp., Sarasota, FL), also operates on the steam/air principle with medium circu- lation achieved by a fan (Lampi, 1979). Although it is important to know the heat transfer char- acteristics of a heating medium before employing it for thermal processing, few studies have evaluated surface heat transfer coefficients (h) of steam/air mixtures. Preliminary Authors Tong, Ramaswamy and Smith are with the Dept. of Food Science, Univ. of British Columbia, Ste. 24% - 2357 Main Mall, Vancouver, B.C., Canada V6T 2A2. Author Stark is with the Re- search Station, Agriculture Canada, Kentville, Nova Scotia, Canada B4N lJ5. studies by Blaisdell(l963), Pflug et al. (1963), Pflug (1964) and Yamano (1976) indicated the steam content (S) of the medium to be a major factor influencing the surface heat transfer coefficient. The medium flow rate, nature of the test material, and mode of condensation of steam have also been identified as important factors (Abdul-Hadi, 1979; Ball and Olson, 1957; Coulson and Richardson, 1977; Kisaalita, 1981; Kusak, 1958; Othmer, 1929). Most of these studies have employed steady state methods, with the sur- face temperature of the test material maintained at 5-20 Co below the medium temperature. Results obtained in this way may not be directly applicable to thermal process- ing of foods since the material surface is within a fraction of a degree of the medium temperature for a considerable part of the process time. An evaluation of the factors influencing surface heat transfer coefficients would be useful in understanding the critical factors involved in steam/air processing. The scarcity of information in this area may be due to the difficulty in using test materials of high thermal conductivity because of unsteady temperatures during the retort come-up period. For this reason, Blaisdell (1963) and Pflug (1964) were limited to conducting their experiments at atmospheric pressure. Reliable estimates of h can only be made using test bricks of high thermal conductivity as discussed by Ramaswamy et al. (1983). Although use of lcw conductiv- ity test materials such as bentonite suspensions (Adams and Peterson, 1982; Yamano, 1976) more closely simulates processing of foods in flexible pouches, the potential heat transfer capability of the heating medium could-be under- estimated. This study examined the effects of the orientation of the test material and the composition (steam content), temper- ature, flow rate and flow direction of steam/air mixtures on the surface heat transfer coefficients in two pilot scale retorts. MATERIALS&METHODS FABRICATION OF TESTBRICKS, gathering of temperature history data and evaluation of surface heat transfer coefficients were carried out according to the procedures described by Ramaswamy et al. (1983). Test bricks Aluminum (Alcan 1000F) and stainless steel (AISI 317) bricks with centrally located Teflon-insulated 24 AWG copper/constantan thermocouples (Omega Engineering, Inc., Stamford, CT) were used in this study. Overall dimensions (cm) were as follows: aluminum, 1.93 x 12.1 x 17.8 (thin) and 3.88 x 12.1 x 17.8 (thick); stainless steel, 1.46 x 12.0 x 17.8 (thin) and 2.58 x 12.0 x 1.78 (thick). The thermal properties of the test materials were as follows: aluminum, thermal conductivity = 239 W/mC, specific heat = 938 J/kgC, den- sity = 2700 kg/ms, and thermal diffusivity = 944 x 10-7 m2/s (calculated) (Smithells, 1962); stainless steel,.thermal conductivity.= 16.2 W/mC (Metals Hand Book, 1980), specific heat = 448 J/kgC (experimentally determined using a Perkin-Elmer DSC-2C Differen- tial Scanning Calorimeter, Perkin-Elmer Corp., Norwalk, CT), density = 7590 kg/m3 (calculated from mass and volume) and ther- mal diffusivity = 47.6 x 10-T m2/s, derived from the previous data. All thermophysical property values were obtained at 100°C which Volume 49 (1984)-JOURNAL OF FOOD SCIENCE-939