In our prior post in this series, "The Impact of Temperature and Pressure on Liner Weight Consistency," we demonstrated that, for a fixed orifice system:
“…the parameters critical to controlling the liner process are temperature and pressure.”
In this segment we will examine the relationship between temperature and pressure to determine their hierarchy in the control process.
How Much Does Ambient Affect Compound?
In that previous post, we also noted that the flow through a fixed orifice is dependent on the combination of the viscosity of the fluid being dispensed and the pressure behind it. We also demonstrated that the typical compound changes viscosity on the order of 15% between 60°F and 90°F – the typical temperature swing of a summer day.
But does that temperature change really affect the compound in the system?
Yes, it does.
We’ve spent hours testing to show that compound temperature at the gun does follow the ambient temperature. But you knew that all along. You’ve watched your operators adjusting pressure – down as the day grows warmer and up through the cool of the night – as they chase the “holy grail” that is the end weight spec you’ve given them.
Temperature Affects Pressure
But temperature and pressure are not independent of one another. Assume that you have a ¼” hose between the regulator and gun. Assuming a constant pressure out of the regulator, the pressure drop in the hose determines the pressure at the gun. That pressure drop is a function of flow rate and viscosity. But we have already shown that the viscosity changes 15% over the course of a typical summer day. That means that the pressure drop in the hose also varies by that same 15% - another reason your operators are constantly adjusting pressure.
A Well-Known Problem
The impact of temperature on liner weight is well known by can makers. This has spawned several homespun approaches to solving the problem. Most involve only heating, though one we’ve run across adds both heating and cooling.
Shown in Figure 1, this system sub-cools the compound as it exits the header, then uses an electrically heated hose to bring it up to the desired temperature on its way to the process. The concept is innovative and recognizes both the need for heating and cooling to address daily and seasonal temperature variations, and the need to manage compound temperature as it travels through the process.
The problem is that it doesn’t go far enough.
Once the compound leaves the heated hose, it is exposed to a constantly changing ambient which changes the temperature, and therefore the viscosity, of the compound before it gets to the gun. As we’ve already demonstrated, this will also impact the pressure at the gun as well.
The result: significant investment – continuously changing liner weights.
A Better Approach
We are devoted proponents of controlling temperature at the point-of- application. This is especially important with the liner application due to the critical balance between temperature, pressure, and dispense volume. It is also important to understand that liner compound, like most sealers, operate best in the normal ambient range (75°F – 95°F). This is where they are both chemically stable and their flow characteristics are optimal for dispensing and placement control. Given that, we knew both heating and cooling capability would be essential to maintain stability in a changing ambient environment.
The Right Solution
We found the solution in a combination of technologies we have employed in other applications. Thermo-electrics offered the option to both heat and cool in a small package that could be mounted close to the gun, without the cost and complexity of a conventional refrigeration system.
Adding a glycol recirculation circuit to our TEC package, allowed us to utilize our various traced hose technologies to extend the “temperature control envelope” all the way to the gun (i.e. – the point-of- application).
Stability Breeds Stability
Stabilizing the temperature of the compound from the outlet of the regulator all the way to the gun means stable compound viscosity. As we discussed, stable viscosity means stable backpressure on the regulator, which improves its performance as well.
The result: a well-behaved, predictable dispensing system.
Simply changing the focus from the compound leaving the header, to the compound at the point of application, makes all the difference in system performance.
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