Çeşitli gıdaların soğutulmasında etkin işlem parametrelerinin belirlenmesi
Determination of the effective process parameters of several food products during cooling
- Tez No: 39141
- Danışmanlar: PROF.DR. OSMAN F. GENCELİ
- Tez Türü: Doktora
- Konular: Enerji, Makine Mühendisliği, Energy, Mechanical Engineering
- Anahtar Kelimeler: Gıdalar, Isı, Parametreler, Ön soğutma, Foods, Heat, Parameters, Pre-cooling
- Yıl: 1993
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
ÖZET Bir soğutma çeşidi olan ön soğutma tanım olarak, meyve ve sebzelerin hasat edildikten sonra pratik olarak mümkün olan en kısa sürede depolama veya taşıma sıcaklığına indirilmesi için yapılan işlemleri içermektedir. Bu işlemin amacı, meyve ve sebzelerin bozulmalarını önlemek ve kalitelerinin uzun süre korunmalarını sağlamaktır. Pratik olarak meyve ve sebzelerin soğutulması uygulamalarında genellikle dört metod kullanılmaktadır. Bunlar, hava ile soğutma, su ile soğutma, vakum ile soğutma ve hava-su karışımlı soğutma sistemleridir. Meyve ve sebzelerin soğutulması sırasında oluşan zamana bağlı ısı geçişinin analiz edilerek ürünler içindeki sıcaklık dağılımlarının ve ısı geçiş karakteristiklerinin (ısıl iletim katsayısı, ısıl yayılım, özgül ısı, efektif ısı taşınım katsayısı, Biot ve Fourier sayıları) ve etkili soğutma işlem parametrelerinin (soğutma katsayısı, gecikme faktörü, yarı soğuma süresi, sekizde yedi soğuma süresi) belirlenmesi, pratik uygulamalarda etkin ve kaliteli bir soğutma işleminin sağlanması, işlem sırasında ürün kalitesinin korunması, işlem süresinde optimizasyon sağlanması ve enerji ekonomikliğinin sağlanması için gerekli olmaktadır. Bu çerçevede deneysel soğutma çalışmaları için meyve ve sebzelerden domates, armut ve salatalıklar seçilerek deney elemanı olarak kullanılmıştır. Bu ürünlere hem su hemde hava ile soğutma teknikleri uygulanmıştır. Su ile soğutma çalışmalarında: bu ürün gruplannın herbirinden homojen olarak 5, 10, 15 ve 20 kg'lık partiler oluşturulmuş olup, bu partiler polietilen kasalara yerleştirilerek soğuk su havuzunda yaklaşık 1°C su sıcaklığı ve 0.05 m/s su akış hızında ayrı ayrı soğutulmuştur. Soğutma işlemi sırasında herbir partide seçilen 12 adet ürünün merkez sıcaklıkları ve su sıcaklıkları ile su akış hızı ölçülmüştür. Bunun yanında aynı boyutlardaki bu ürünlerden beşer kg'lık partiler oluşturularak polietilen kasalara yerleştirilmiş, bu partiler değişik hava hızlarında (1, 1.25, 1.5, 1.75 ve 2 m/s) ve yaklaşık 4°C'lik hava sıcaklığında soğutma kabininde soğutulmuştur. Bu sırada her partideki 12 adet ürünün merkez sıcaklıkları, üründeki kuru madde değişimi, partilerdeki ağırlık değişimi, ortam sıcaklık ve nemi ve hava akış hızlan ölçülmüştür. Ürünlerin hem su hemde hava ise soğutulması sırasında ölçülen sıcaklık dağılımlarına üstel formda regresyon analizi uygulanmıştır. Buradan elde edilen etkin soğutma işlem parametrelerinin yanısıra ısı geçiş karakteristikleri de belirlenmiştir. Ayrıca bu çalışmada geliştirilen bir model yardımı ile efektif ısı taşınım katsayıları oldukça basit bir şekilde belirlenmiştir. Bu değerlerin pratikde ölçülmesi oldukça zordur ve buna benzer bir çalışmaya literatürde rastlanılmamıştır. Bulunan efektif ısı taşınım katsayılarının kullanımıyla teorik olarak hesaplanan soğutma yükü dağılımları ile deneysel soğutma yükü dağılımları karşılaştırılmış ve oldukça iyi bir uyum bulunmuştur. Ayrıca yine bir uygulama olarak, bütün deneysel şartlar için herbir ürüne ait teorik sıcaklık dağılımları efektif ısı taşınım katsayılarının kullanımı ile hesaplanmış ve deneysel ölçümlerden elde edilen sıcaklık dağılımları ile karşılaştırılmıştır. Bu karşılaştırmalar sonucunda çok iyi bir uyum bulunmuştur. Sunulan modeller sonucu elde edilen efektif ısı taşınım katsayıları kullanımı ile yeni efektif Nusselt-Reynolds eşitlikleri bulunmuştur. Ayrıca, deneysel yan soğuma ve sekizde yedi soğuma sürelerinin Reynolds sayısına göre değişimleri verilerek, literatürde olmayan yan soğuma ve sekizde yedi soğuma süreleri için Fourier-Reynolds eşitlikleri geliştirilmiştir. xıı
Özet (Çeviri)
DETERMINATION OF THE EFFECTIVE PROCESS PARAMETERS OF SEVERAL FOOD PRODUCTS DURING COOLING SUMMARY Precooling is a cooling operation in which the field temperature of the fruits and vegetables is reduced to the storage and/or transportation temperature in the shortest time possible after harvest in the field in order to prevent spoilage and to maintain all possible preharvest freshness and flavor. This term implies the removal of the field heat before processing, transporting or storing. Cooling inhibits growth of decay-producing microorganism and restricts enzymatic and respiratory activity during the postharvest holding period, inhibits water loss, reduces product ethylene production, and reduces sensitivity of products to ethylene. The holding period may be the relatively short time required to transport and sell or process the product, or it may include a long-term storage period as well. In recent years, increasing interest has been focussed on the determination of the effective cooling process parameters (cooling coefficient, lag factor, half cooling time, seven- eighths cooling time), heat transfer characteristics (thermal conductivity, thermal diffusivity, specific heat, surface heat transfer coefficient, Biot number, Fourier number) temperature distributions and heat transfer rates (cooling heat load) in perishable produce, particularly in fruits and vegetables during cooling applications. Application of cooling plays a key role in the storage and transport of fruits and vegetables. Its importance becomes even more pronounced in Turkey which is a country with a high potential for fruit and vegetable production. It is, therefore, important to determine the effective process parameters for quality, processing and economic reasons and provides optimum processing condition and energy saving. Several cooling methods, such as forced-air cooling, hydrocooling, vacuum cooling and hydrair-cooling are widely used in practice. For the cooling of fruits and vegetables, the technique used is often determined more by factors such as economy, convenience, effectiveness, utilization of equipment, operation conditions, and personal preference. Fast cooling can be achieved in an air blast tunnel and high speed jet cooling. With these methods there is always a risk of removing to much moisture from the product and consequently other methods of chilling have been developed, notably hydrocooling by immersion in chilled water. Vacuum cooling has also been developed for vegetables, in particular leaf crops. The larger the surface of the product the more successful is this method of cooling. Hydrair-cooling utilizes a mixture of cooled air and water in the spray that is circulated around and through the stacks by forced convection. Hydrocooling is an effective method of quickly removing field heat and has important advantages due to its simplicity, economy and effectiveness. In the food processing, there are many situations where the temperature at any point in a product is a function of time, i.e., transient heat transfer is occurring. The most notable examples of transient heat transfer are heating, cooling, precooling, freezing, drying xiiiand blanching of the food products. In the transient heat transfer during hydrocooling of food products, the temperature at a given point within the food depends on the cooling time and the position in the food. The factors influencing the temperature change are the temperature and flow rate of coolant, the thermal properties of the product, and the product's size and shape. Regardless of the type of hydrocooling technique, knowledge of the thermal properties is essential for determination of the product thermal characteristics and for the successful development of the necessary technology base. For this reason, estimation of the effective heat transfer coefficients plays an important role in order to carry out optimum processing conditions and energy saving and to allow an exact analysis of the process heat transfer in these applications. In order to establish the mathematical model for the support of the experimental observations, the boundary condition of the third kind in the transient heat transfer is considered for Biot numbers between 0 and 100. It is the most realistic case due to containing both the internal and external resistances the heat transfer from the products. Consider the cooling of a solid spherical or cylindrical product of radius R immersed in water at constant temperature Ta, with a constant convective heat transfer coefficient h for both products. Att=0, temperature distribution is assumed to be given. This conduction problem in the spherical and cylindrical systems involves the spherical symmetry and axial symmetry exist. Assuming a homogeneous and isotropic solid sphere and cylinder, constant thermal properties, uniform initial temperatures, constant medium temperature, constant heat transfer coefficients, negligible internal heat generation, and heat conduction in the radial direction only. Consider constant thermal and physical properties of the product and the cooling medium when operating under the unsteady-state conditions. The following analytical formulation is used as the technique for analyzing the cooling data during the cooling of food products. Dimensionless temperature is expressed using the product temperatures and the medium temperature, respectively, 0 = Cr-Ta)/(TrTa) (1) The dimensionless temperature is generally expressed in the form of an exponential equation, including the cooling parameters in terms of the cooling coefficient (C), and lag factor (J), as 6 = Jj-expf-C-t) (2) The cooling coefficient denotes the change in product temperature per unit change of cooling time for each degree temperature difference between the product and its surroundings. xrvBy substituting 8=0.5 into Eq.(2), the half cooling time, which is one of the most meaningful in the practical applications, is defined as Z = [ln(2J)]/C (3) Also, by substituting 6=0.125 into Eq.(2), the seven-eighths cooling time is found as S = [ln(8J)]/C (4) Mathematical formulation of this heat conduction problem in the spherical and cylindrical coordinates for both products may be written in the following general form: (d*T/dr*) + (Z/r)(dT/dr) = (1/a)(3T/a) (5) The formulation in terms of the excess temperature <j> = T-TQ is (dtydfi) + (Z/r)(d$/dr) = (1/aK4>T/ft) (6) where Z=1 for cylinder and 2 for sphere. The boundary and inlet conditions for both the spherical and cylindrical body, in dimensionless form, are <(>(r,0) = 4>/ = T/-Ta (7) 4>(0,t) = finite (8) (Ö<J)(R,t)/ör) = -(h4>(R,t)/k) (9) The solution of the above equations can be made using general techniques, e.g. laplace transform and separation of the variables as given in literature. Thus, the transient temperature distributions for the spherical and cylindrical products are resulted in the form of the following equation: n=1 where 2-Big-SinN^ (SinNn-r) \ ? Bn = e*P(-V-R>s), Cn = (N“ - SinNn-CosNn) (Nn-r) for sphere. 2-Bic An =, Bn = exp(-Mn*.Foc), C”= JQ(Mn-D Jo(Mn)-[Mn2 + Bic2) for cylinder. xvEq.(10) permits to estimate the temperature at any point (center, half radius and surface) of both spherical and cylindrical bodies. For the center position, Cn=1 due to r=0. The Biot and Fourier numbers and dimensionless radial distance are defined as Bi = h-R/k (11) Fo = a<t/R2 (12) T = r/R (13) At the center of the infinite cylinder and of the sphere, r=0. When Fo>0.2, the infinite sum can be approximated by the first term of the series in Eq.(10) and may be represented by the following expressions 0C = A7 c-exp(-M7*-Foc) (14) es = A^'s-exp(-N72-Fos) (15) Applying regression analyses using the least squares method, the dimensionless temperature distributions for both the cylindrical and spherical products are obtained in exponential form as ec = J7c-exp(-Cc<t) (16) 6S = J7's-exp(-Cs-t) (17) The following equations are obtained by equating Eqs.(14), (16); and Eqs.(15), (17) in the consideration of A^=Jy. M72-Foc = Cc-t (18) Nr2-Fos = Cst (19) The values of M^ and N^ are determined by means of the following characteristic equations N^-CosNy = SinNf-(1 - Bi) and M^-J^Mj) - Bi\J0(Mj) = 0. These characteristic equations can be simplified as given below!t300R W1 = (6Bic)/(2.85 + Bic) (20) N27 = (10.3Bis)/(3.2 + Bis) (21) The experimental temperature distributions of both spherical and cylindrical products in dimensionless form are observed by using measured temperature values in the following equation and these temperature distributions are used in the regression analyses. e = cr-Ta)/crrTa) (22) The following models are developed to determine the surface heat transfer coefficients for spherical and cylindrical bodies subjected to cooling after the required substitutions. hs = (3.2-ksfls-Cs)/(10.3-as - Cs-Rs2) (23) hc = (2.85.kcflc-Cc)/(6-ac - Cc-Rc2) (24) The thermal properties of food products strongly depend on their water contents. Therefore, the thermal conductivity and thermal diffusivity are estimated by using the following Sweat and Riedel correlations. xvik = 0.148 + 0.493-W (25) a = 0.088-10'6 + (a^ - 0.088-10"6)W (26) The parameters, such as cooling coefficient, half cooling time, seven-eights cooling time and lag factor are the parameters for evaluating and presenting a cooling process. Half cooling time was the most meaningful of the parameters used because it describes the cooling rate in terms of the time required to reduce product temperature by one-half of the difference in temperature between the product and cooling medium. The cooling coefficient, C, denotes the change in the product temperature per unit change of cooling time for each degree temperature difference between the product and its surroundings. These coefficients were obtained by means of regression analyses in the experimental form. The lag factor is a function of the size and shape, and the thermal properties, such as the effective heat transfer coefficient, thermal conductivity, and the thermal diffusivity of the product being cooled. The lag factor exhibits the resistance to the heat transfer inside the product. The cooling rate of the product is a function of the boundary layer surrounding the product. The boundary layer around the product is often characterized in the form of a heat transfer coefficient referred to as the product and fluid interface effective surface heat transfer coefficient. In order to determine the optimum thermal process this effective heat transfer coefficient must be determined either experimentally or theoretically, and it must approximate the actual value closely. Many of the studies are found in the literature deal with the determination of the temperature distributions of the food products during cooling and several literature surveys on the heat transfer coefficients in the food refrigeration processes have been conducted as a search report. The results of this review indicated a lack of experimental data related to the effective heat transfer coefficients of the food products. In this respect, an experimental and theoretical investigation was conducted to determine the effective cooling process parameters and heat transfer characteristics for the food products, namely tomatoes, pears and cucumbers during both air and water cooling. However, no similar study for arbitrary or geometrical shaped food products appeared in literature. The cooling loads (heat transfer rates) and temperature distributions in dimensionless form were estimated using the effective heat transfer coefficients obtained from the present modelling and compared with the experimental measurements. Additionally, the new Nusselt and Reynolds correlations and Fourier-Reynolds correlations were developed to use in practice. The main objective of the present study was to analyze the transient heat transfer and the cooling data in order to determine effective cooling process parameters and heat transfer characteristics of the individual products during the hydrocooling of tomatoes, pears and cucumbers in crates containing 5, 10, 15, and 20 kg of product, in a cooling water environment at 1°C and at the flow velocity of 0.05 m/s and during the air cooling of tomatoes, pears, and cucumbers in crates containing 5 kg of product, in a cooling medium at the temperature of 4°C and at the flow velocities of 1, 1.25, 1.5, 1.75, and 2 m/s, respectively. In order to carry out experiments, an immersion type hydrocooling system, which consists of a conventional vapor-compression refrigeration unit and a cooling pool for food- cooling in water flow, and a forced-air cooling system, which includes a refrigeration unit and a cooling chamber for food-cooling in air flow, was used. These cooling systems were xviidesigned and installed in the Laboratories of the Food and Refrigeration Technology Department, TUBITAK-Marmara Research Center. Batches of 5, 10, 15 and 20 kg for water cooling and the batches of 5 kg for air cooling were selected from three food commodities, namely tomatoes, pears and cucumbers as perfectly spherical and cylindrical products and placed into the polyethylene crates (open cases). Then, each batch was cooled and the temperatures of twelve spherical or cylindrical products in each batch were measured and averaged for data analysis. The cooling parameters in terms of lag factor, cooling coefficient, half cooling time and seven-eighths cooling time were determined from the exponential decline in the dimensionless temperature at the centers of the individual products. The cooling parameters were found to be dependent on the batch weight (crate load) in hydrocooling and on the flow velocity in air cooling. The results of this study indicated that the half cooling and seven-eighths cooling times increased and the effective heat transfer coefficients of the individual products decreased, with increasing crate load in hydrocooling and that the half cooling and seven-eighths cooling times decreased and the effective heat transfer coefficients increased, with an increase in the flow velocity of air. The variations of the theoretical heat transfer rate (cooling load) of both spherical and cylindrical products were predicted using the effective heat transfer coefficients in the heat transfer modelling. These cooling load variations were compared with the variations of experimental heat transfer rate in a very good agreement form. XVIII
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