Flow rate calculation of coating application

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Discussion Overview

The discussion focuses on calculating the flow rate of a coating application from a pressure tank through a dispensing valve. Participants explore theoretical approaches to determine flow rate, including the use of the Hagen-Poiseuille equation and Bernoulli's principle, while considering factors such as pressure, viscosity, and the geometry of the dispensing system.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant suggests using the Hagen-Poiseuille equation for calculating pressure drop in the hose but questions its applicability for the dispensing valve due to its non-constant opening.
  • Another participant recommends the "Energy Equation" for incompressible flow, emphasizing the need for detailed information about the valve and plumbing components.
  • There is a claim that flow rate is expected to be linear with respect to pressure, while another participant proposes a quadratic relationship with the radius of the dispensing valve.
  • Concerns are raised about fluctuating flow rates, with participants questioning whether this could be due to viscosity changes, tank substance settling, or temperature variations.
  • Some participants discuss the implications of Newtonian versus non-Newtonian fluid behavior, particularly regarding how viscosity might change with shear rate or time.
  • One participant mentions that the nominal dynamic viscosity of the coating is 1.0 Pa.s and expresses uncertainty about the effects of fluctuating viscosity on flow rate.

Areas of Agreement / Disagreement

Participants express differing views on the relationships between flow rate, pressure, and viscosity. While some agree on certain theoretical relationships, there is no consensus on the impact of viscosity fluctuations or the exact nature of the flow behavior in this specific application.

Contextual Notes

Participants note limitations in their ability to measure viscosity due to the reactive nature of the coating and the potential influence of external factors on flow rate. There are also unresolved questions regarding the applicability of different fluid dynamics equations in this context.

  • #31
bertcoen said:
curing mechanism of the coating is a humidity reactive system.
Two possible ways I see this could cause your variations:

1) The spray nozzle is not being cleaned at end-of-shift and the residual compound is curing in it during the night.
2) Air used to pressureize the tank is moist causing partial curing. Air right out of a compressor is notoriously wet.

#1 has an obvious solution.
#2 As a test, get a tank of Dry Nitrogen with appropriate pressure regulator. Purge the reservoir,hose, nozzle, etc. and try that for a few days/weeks.

If #2 solves the problem, consider a dryer for the air line (expensive), perhaps a dessicant canister (periodic maintenance required) in the air line, or continue with the Dry Nitrogen tank.

Cheers,
Tom

p.s. As a last resort, farm out the coating process to someone else with a well-written/comprehensive quality requirement. :rolleyes: The downside (beyond cost) is the incoming quality-control inspections needed.
 
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