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Roll Heat Transfer Coefficients
March 7, 2008
Last time I spoke about two cast film lines with different chill roll heat transfer coefficients which were affecting the film properties developed on the two different cast lines. So what controls the heat transfer of a chill roll? Primarily the roll heat transfer is controlled by the sum of the resistances to heat flow from the polymer on the roll surface to the water (or other coolant) flowing in the roll. This is essentially the thermal conductivity of the shell material, the convective heat transfer rate of the water to the shell surface and any fouling factor. Figure 1 shows the typical construction of a spiral heat transfer roll typical of modern chill rolls.
In order to calculate the heat transfer coefficient you need to know the roll shell material and thickness as well as the dimensions of the spiral flow channels and the water flow rate in the roll. From this information you can calculate the water velocity in the channel and then using available heat transfer relationships calculate the water to shell heat transfer coefficient. Then in combination with the thermal conductivity estimate the overall heat transfer coefficient of the roll. This is then used to determine the capacity of the roll (line speed at a design stripping temperature) or the temperature of the film at any given line speed.
But knowing all of this information about the roll internal design can be hard or impossible to know as oftentimes prints are not available, or the prints available are not for the roll in question. Without this information can we still determine the heat transfer coefficient? Well yes, we can measure the roll heat transfer coefficient using the relationship:
Q = h (Tpolymer-Twater)
Where Q is the heat flux (BTUs/ft^2 or Kwatts/M^2), h is the overall heat transfer coefficient, Tpolymer is the temperatures of the molten polymer applied to the roll, Twater is the average temperature of the water in the roll. If we know Q the heat flux and the polymer and water temperatures we can easily calculate H the overall heat transfer coefficient. Fortunately we can determine the value of Q by a couple of methods. First is using the temperature rise in the water in the drum and second is using the heat loss of the film. To measure these we need to know the mass flow rate of the water (Wmass)and the inlet and outlet temperatures of the water, or the mass flow rate of the polymer (Pmass) and the melt temperature (Tmelt) and the temperature at the removal point of the roll (Tstrip). The flux is calculated by dividing the heat change (Q) by the area of the transfer which is the area of contact between the film and the roll surface (Ac)
Q = Wmass(Tout-Tin)/Ac
Q= Pmass (Tstrip - Tmelt)/Ac
So we can equate and rearrange to find h
h = [Wmass(Tout-Tin)/Ac ]/ (Tpolymer-Twater)
Posted by Eldridge M. Mount on March 7, 2008 | Comments (0)