Effect of change in ambient temperature
Flash cure systems are tested in a normal ambient environment of about 74°F to determine air flow parameters to maintain IRt/c sensor body within its specified limits, as well as other cooling and ventilation needs in the machinery. However, in hot climates in other parts of the world, machine room ambient is expected to be as high as 105°F. How does this affect the cooling air requirements?
Assuming nothing in the machinery changes in dimensions or other characteristics when the ambient temperature changes, the only significant effects of the higher ambient air temperatures are:
1. Lower density of the air flowing through the machine at 105°F compared to higher density at 74°F.
2. Less heating power (or on time) of the heating elements to raise the T-shirt to its 450°F setting. This assumes the T-shirts are at ambient temperature before printing and curing. (Note, if Speed Boost principles are employed to increase throughput rates, then this effect may disappear.)
Effect 1. The air density has no effect on the air volume flow through the system: the blower produces the same cfm, and the system resistance is the same. However, the lower air density at the higher temperature reduces the mass flow of air, and therefore the amount of heat that the air can carry away. The density ρ change is inversely proportional to the absolute temperature. For the conditions T1 = 74°F and T2 = 105°F,
ρ1/ ρ2 = (460+T2)/(460+T1) = (105+460)/(74+460) = 1.058; air is 6% less dense at 105°F
The heat transfer coefficient is given by the Nusselt No. (Nu) which changes with air density to the power 0.8. Accordingly,
Nu1/Nu2 = (ρ1/ ρ2)0.8 = (1.058)0.8 = 1.046; air carries away about 5% less heat at 105°F
Effect 2. Since less lamp heat q is required for the shirt to reach 450°F when starting at 105°F compared to 75°F,
q1/ q2 = (450-74)/(450-105) = 1.090; about 9% less lamp heat is needed at 105°F ambient air
The net result of the two effects is that at 105°F ambient there is about 9% - 5% = 4% less temperature rise of the sensor body than at 105°F ambient air.
Accordingly, if the tests at 74°F ambient show that the sensor body increases from 74°F to 115°F after the machine is stable, then the ∆T is 41°F. Then at 105°Fambient, the ∆T = 0.96 x 41 = 39.4°F, and the sensor body will be at 105 + 39.4 = 144.4°F.
For the power loss condition, the residual heat effect will produce nominally the same temperature rise of the sensor at 105°F ambient as observed at 74°F ambient. There may be a reduction of up to 9% of this residual heat rise due to Effect 2, the lower average lamp power usage when ambient is 105°F. Accordingly, if the residual heat rise is 40°F when the ambient is 74°F, then the rise at 105°F ambient will be in the range 0.91 x 40 ≈ 36°F, to 40°F. The smaller number is somewhat uncertain because it depends on the dynamic thermal status of the lamp system, so it is best to stay with the higher figure for the temperature rise due to the residual heat at power loss.
In summary, if the test data at 74°F indicate a sensor body temperature of 115°F, then the sensor body temperature will be 115 +34 = 144°F during operation at 105°F, and 144 + 40 = 180°F for the power loss condition at 105°F.
This may be too close to the 212°F sensor body limit for good field robustness. Accordingly, there are several things that could be improved to reduce the 180°F by 10°F, for example:
a) Increase the cooling air to the sensor by approximately 25% (example, 10°F /(115°F -74°F) = .24).
b) Increase the surface area of the sensor holder block of material that is exposed to the cooling air, by about 25%.
c) Increase the mass of the sensor holder block of material by about 33% (example, 40°F /(40°F -10°F) = 1.33
They are all additive, so any combination of smaller changes will also work to reduce the maximum sensor body temperature.
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