know_your_flow

 
ONE PIECE OF INFORMATION MOST OFTEN MISSING IN MOST PROCESSES TODAY IS...
RATE OF COOLANT FLOW!
Here's a primer on why knowing your flow can cut your costs, and how to measure that flow rate.
BY
Ron Wolfe, Vice president, Technology, Advantage Engineering, Inc.
 
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FOR MAXIMUM PRODUCTIVITY...

but there is another key piece of information that is missing in 95% of all processes running today, and that is the flow rate of the temperature control medium.
WHAT MAKES THE FLOW RATE OF THE LIQUID MEDIUM GOING TO YOUR PROCESS IMPORTANT?
Why should you have a flow meter measuring it so it is always known? Obviously, the flow of coolant through the mold is vital to the molding process.
It is widely known that the best heat transfer occurs with a
turbulent flow. This is indicated by a high Reynolds number, which can only be calculated if the actual flow rate is known. Consequently, the process flow rate should not only be known, but also be communicated via SPI protocol to a central computer in the molding area where a low-flow alarm condition can generate the necessary attention. Beyond this, here are 10 simple, no-nonsense arguments for getting to know your flow.
1. Make sure that piping and manifolding arrangements are correct. Some methods of connecting mold temperature controllers to molds result in a virtual spaghetti of hose and couplings that ravenously consumes pump pressure. Forcing a pump to overcome these losses will result in reduced flow. Many molders are surprised to learn that even a few minor changes in plumbing can greatly increase flow to the tool and decrease cycle times. The only way to evaluate the benefit is with a flow meter.
2. Don't buy too much pump. Many molders buy big pumps in portable equipment mainly because they don't know what flow rate they need, and they figure 'more is better'. This may be true in some cases, but if the geometry of the piping, manifolding and tool will not allow more flow than that produced by a smaller pump, why pay more? The average cost of upgrading a mold temperature controller pump from 3/4 to 3 hp is $300. If the average shop has 15 presses and 30 controllers the cost is $9000. Not exactly a windfall, but the best way to save $100 is to find 100 places to save $1.
3. Discover fouled lines or tool passages. If the known flow rate of a given process begins to decrease over a period of time, usually the diagnosis is fouling. A prominently displayed flow rate can make this obvious so the problem can be dealt with before it becomes a disaster.
4. Manage energy. This is a hot topic these days, and many dollars are being spent on premium-efficiency motors. The motor itself, however, is only half of the story. The benefit of a motor operating at 97% efficiency is diminished if it is coupled to pump that is operating in the 55-60% efficiency range. A flow meter can help ensure that the system operates at its peak combined efficiency.
5. Assure that the pump is rotating in the proper direction. Don't laugh. Even though a flow meter makes a pretty pricey phase detector, most people would be shocked to learn how many pumps are found running backwards in the midst of expensive service calls.
6. Troubleshoot the entire temperature control system. Often when a flow problem exists in a process, you don't know whether the pump or the process is causing it. By separating the two, and using a flow meter as a diagnostic device, it's possible to zero in on the problem more readily.
7. Take 'snap shots' of thermal capacity at various stages of cycle. If the flow rate is known, and the temperatures to and from the process are known, comparing the actual heat rejection to the theoretical values becomes a simple exercise. The following formulas can be used to calculate required tonnage of cooling required for a given temperature change, or conversely the amount of heat/hr that must be removed to achieve required temperature change:

Tons of Cooling=(gal/min x °F temperature change)/24
BTU/hr=gal/min x 500 x °F temperature change

Becoming familiar with this kind of information will take some of the guesswork out of future equipment purchases. (The formulas above are not quite accurate for fluids other than water, but are close enough for the intended purpose.)

8. Recreate minimum cycle time setup for a given tool. Shops that change tools frequently can keep a record of the optimum flow rate for each tool, and when changing tools, quickly and easily dial in the flow rate that produced the highest quality parts at the minimum cycle time the last time that tool was used.
9. Throttle the pump. Many systems are designed in such a way the the pump should not be run 'wide open'. Knowing the optimum system flow rate, a flow meter will help you throttle the pump appropriately to keep it from overloading.
10. Develop an intuitive knowledge of flow in systems. People who work around flow meters develop a sixth sense about flow rates. Developing a sense for flow rate by 'eyeballing' a valve handle or listening to the sound that a by-pass valve makes can come in handy in a situation where no flow meter is available.
SELECTING THE RIGHT FLOW METER
Now that it has been established why the flow rate of process coolant is important, it might be a good idea to examine how it can be measured. What types of flow meters are suitable for the average process, and what are the relative costs?
Two things to keep in mind before making any decisions about flow meter selection: 1) All flow meters will be adversely affected by poor coolant quality, though some types more than others. And 2) Always choose a meter with a realistic scale. For instance, if the estimated or expected flow rate is 3 gpm, don't buy a meter that is calibrated for zero to 300 gpm.
Most flow meters fall into three broad categories: float type, turbine type and electromagnetic.

As a processor you must continually look for ways to improve productivity, decrease cycle times, and tighten up your operation to remain competitive and improve profitability. Chillers and mold temperature controllers can play a significant role in the quest for increased productivity.
Manufacturers of such equipment provide an array of features designed to help you use their equipment properly and to achieve maximum efficiency. These include constantly displayed set points, 'to' and 'from' process temperatures percent capacity, SPI communication protocols, programmable alarms and dual pressure gages. These are all helpful,

figure_two
1. The float-type meter (fig 1), also referred to as a rotameter, operates on the principle of variable area. The fluid flow raises or pushes a weighted float up through a tapered tube which increases the area for the fluid to pass.
For each flow rate the float will reach stable position in the tube where the upward force created by the fluid is equal to the downward force of gravity acting on the float.
The function is linear and is both accurate and very repeatable.
The rotameter is popular because it is relatively inexpensive, about $150-$300, and readily available. Some people do not prefer it because it is 'low tech'.
 


2. The turbine-type meter (Fig 2) is available in a huge array of mechanical configurations, but basic operation is pretty much the same. A rotor, paddle wheel, or some other propeller shaped device, is mechanically held in the fluid flow stream which causes it to spin
The blades of the rotor are magnetized so that alternating poles pass by the Hall Effect sensor when the rotor is in motion. The controller converts the alternating poles seen by the sensor into revolutions, and subsequently, since the cross section of the pipe is known, into velocity and flow rate. Turbine meters tend to be moderate in cost, about $300-$1000, easily calibrated in the field, and are available in size and material of construction to fit just about any application.

3. Electromagnetic flow-type meters (Fig 3) operate on Faraday's Law of Magnetic Induction. This law states that a voltage will be induced into a conductor that moves in a magnetic field. In this case, the conductor is the flowing fluid, and the induced voltage, which is sensed by the two electrodes immersed in the fluid, is proportional to the flow velocity. Because the cross sectional area of the pipe is known, the velocity can be easily converted to a flow rate. Electromagnetic flow meters are the meter of choice where precision and reliability are major considerations. Also, there are no moving parts to be fouled by contaminated water. Meters in this category are consequently at the extreme high end of the cost scale, unusually about $2500 and higher.