Three technologies are used to cool canned products: tunnel, rotary and spin cooling. The basic difference among them is the level of agitation or mechanical mixing imparted to the contents of the container as it is processed. Testing has shown that heat transfer rates are increased significantly with constant mechanical mixing.
Thermal processing of canned foods is an example of transient heat flow because the temperature of the contents of the container is changing as a function of time. Both cooking and cooling are transient heat flow but in opposite directions. When a mass a verage temperature is considered, heat transfer in or out of most canned products can be predicted using the following equation:
Tt = Te + (Ti - Te) e-rt
Ti is the i n itial product temperature (oF)
Tt is the product temperature (oF) at time (t)
Te is the environment temperature (oF), which is constant through time (t)
t is the time (min)
r is a rate c on stant (min-1)
The "r" factor is dependent on product properties such as specific heat, viscosity, thermal conductivity, convective heat transfer coefficients and density as well as container properties such as surface area, volume, shap e and material. Because it is difficult to calculate r values, they usually are found through laboratory testing or field data. It is critical to understand that r is not a function of either product temperature or environment temperature. Once it is dete rmined, it can be used to accurately predict the cooling performance at any initial temperature, water temperature or time if all other factors remain constant (see sidebar).
From the sidebar, it is clear that using the coldest water available t o inc rease cooling rates is advantageous, but this is not always within the processor's control. The processor must accept the water at the temperature it is delivered from the well or city source. The use of cooling towers is common but delivers even warmer w ater, especially in the summer months. Water chiller systems trade electrical energy for thermal energy and, except in rare circumstances, are too expensive.
Mechanical MixingMechanical Mixing
One factor that affects heat transfer rates is convection or mechanical mixing of the container contents. Product mixing helps to break up the hot core of a container and brings the heat to the outer part, where it can be removed by convection with the water sprays. This same principle is applied with heating by stirring a pan on the stove to prevent the bottom material from scorching. Products with low viscosity such as light juice products develop natural convection currents within the container as a temperature differential forms. The cooling produced by the convection currents is similar to mixing but less effective. In products with higher viscosity such as sauces and purees, no convection currents can form. Without mechanical mixing, the heat transfer can be no faster than the rate of conduction from the core to the outer part of the container.
Unfortunately, the choice to increase cooling rates by mechanical mixing must be made at the time of equipment purchase. Currently, the basic types of coolers are tunnel, rotary and spin. Each provides a different level of mechanical mixing and heat transfer efficiency.
A tunnel cooler consists of a conveyor belt on which containers are loaded closely packed and standing upright. The conveyor passes through a series of cascading water zones, and the containers are cooled as the water picks up the heat. No mechanical mixing occurs during the cooling process.
A rotary-type cooler uses the principle of intermittent agitation to provide more rapid heat transfer. The cooler consists of a stationary cylindrical shell that contains a rotating reel with steps to hold conta i ners. A spiral track system is fixed permanently to the inner surface of the shell. The turning of the reel inside the shell and the lead of the spiral advance the containers along the perimeter and length of the shell. Container rotation can be divided i nto three phases: fixed reel, sliding rotation and free rotation (figure 1). During the free-rotation phase, the containers roll freely along the inside bottom wall of the shell. The majority of product agitation occurs during this phase. During the other phases, the containers roll at approximately the same rotational speed as the reel, which usually is in the range of 3 to 8 rpm.
It is important to mention headspace and its cri tical effect on mixing and heat transfer rates. Headspace is the volume in the container that is void of product. Headspace control is critical to the cooling process if mixing is to occur. The can must contain an air bubble that will allow the product to tumble slightly as it rotates; otherwise, the product rolls as a mass and no mixing occurs.
q = (m/t) x Cp x (Tf - Ti)
q is the heat transfer rate (BTU/min)
m is the mass (lb)
t is the time (min)
Cp is the product specific heat (Bt u/lb-oF)
Tf is the product final temperature (oF)
Ti is the product initial temperature (oF)
Always use mass average or shake temperatures for heat transfer calculations. Using center-can temperatures will g i ve erroneous results.
Assuming a desired discharge temperature of 105oF (41oC), the required times can be taken from the cooling curves. One calculation is shown for illustration; the remainder is in table 1.
q (juice, s ta tic) = (2.875 lb/5 min) x 1 BTU/lb-oF x (203oF - 105oF) = 56.35 BTU/min
From these values, one can calculate that for the light juice product, the spin cooling technology gave a rate of heat transfer 2.4 times that of static cooling and 1.8 times that of rotary cooling. For the thicker tomato product, the rate for spin cooling is 6.3 times that of static cooling and 2.3 times that of rotary cooling.
These results demonstrate that the spin cooling technology giv es the greatest heat transfer rate of the three technologies evaluated. They also demonstrate that the advantage of spin cooling vs. other technologies is greatest when used with products that have higher viscosity.
Testing has shown that as the rotat ional speed is increased from zero, the rate of heat transfer in most products increases almost linearly until a critical speed is reached. At this point, the heat transfer rate becomes almost constant as rotational speed continues to increase until a sec ond critical point is reached. At this second critical point, which corresponds to centrifugal force preventing the product from tumbling in the container, the heat transfer rate drops quickly back to the rates found in static conditions. Spin coole rs are sized to provide rotational speeds that fall within the limits of these two critical points.
What does this mean to the processor? One conclusion is that the more efficient process will cost less to operate. In the tests, heat transfer efficien cy was demonstrated by the greater rate of heat removal with the same consumption of pump horsepower and water flow rate. Also, product quality is likely to be higher if overcooking is arrested quickly in the cooling process, which results in better color and c rispness of the product.
Choosing a cooling technology that uses the highest degree of mechanical mixing will shorten process times, increase efficiency and throughput, and possibly enhance product quality. The advantages of spin cooling are de monstra ted for low viscosity products but are most dramatic in heat transfer rates of highly viscous products such as sauces or purees. Although the focus here has been cooling, the same conclusions can be applied to heating as both heat transfer processes are governed by the same physical laws operating in reverse. PCE
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Sidebar: Predicting Cooler PerformanceA can of product was cooled in the laboratory from 200oF (90oC) to 100oF (38oC) using 60oF (16oC) water in 20 min. How long will it take to perform the same cooling using 80oF water? First, solve the cooling equation for r,
r = -(1/t) ln [(Tt - Te)/(To - Te)]
= -(1/20) ln[(100 -60)/(200 - 60)]
Now solve the cooling equation for t, and use the new Te of 80oF (27oC)
t = -(1/r) ln [(Tt - Te)/(To - Te)]
= -(1/0.0626) ln[(100 -80)/
(200 - 80)]
= 28.6 min
You see that it will take nearly 50% more time to cool using the warmer water.
Once the r factor is determined, it can be used to accurately predict the cooling performance at any initial temperature, water temperature or time if all other factors remain constant. Calculations such as these are the basis for predicting cooling performance on industrial cooling equipment or sizing the machinery for the intended production rates.