11 Reasons to Know Your Flow

Using a flow rate to monitor your process cooling water flow will help you improve productivity and minimize downtime.

Plastics processors must always be on the lookout for ways to improve productivity, decrease cycle times and tighten up their operation to remain competitive and improve profitability. Various "beside the press" auxiliary equipment -- primarily chillers and mold temperature controllers -- are tapped as big players in the quest for increased productivity. The manufacturers of such equipment have responded by providing an array of features, including constantly displayed setpoints, to and from process temperatures, percent capacity, SPI communication, programmable alarms and dual pressure gages. All of these features are helpful, but another piece of information can help you improve your productivity even more -- if you know it. That piece, which is missing in 95% of all processes running today, is the flow rate of the temperature control medium.

What makes the flow rate of the water going to your process so important, and why should you have a flow meter measuring it so it is always known? Obviously, the flow of cooling water through the mold is vital to the molding process. Disastrous things can happen when flow is interrupted intermittently or dips below a certain rate. It also is widely known that the best heat transfer occurs with a turbulent flow. This is indicated by a high Reynolds number, which only can be calculated if the actual flow rate is available. Clearly, not only should the process flow rate be known, but it should be communicated via SPI protocol to a central computer in the molding area, where a low-flow alarm condition can generate the necessary attention.

If that is not reason enough, here are 10 more no-nonsense ways that knowing your flow rate will benefit your process.

Corrects piping and manifolding arrangements that may be costing performance.

Some methods of connecting mold temperature controllers to molds result in a virtual spaghetti bowl of hose and couplings that ravenously consumes pump pressure. Forcing a pump to overcome these losses will result in reduced flow. Even a few minor changes in plumbing can greatly increase flow to the tool and decrease cycle times.

Helps eliminate buying too much pump.

Many molders buy big pumps in portable equipment -- mainly because they do not 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 under normal conditions will not allow more flow than that produced by a smaller motor, why pay more? The average cost of upgrading a mold temperature controller pump from 3I4 to 3 hp is $300. If the average shop has 15 presses and 30 mold temperature controllers, that comes to $9,000. Granted, this potential savings is not exactly a windfall, but the best way to save $100 is to find 100 places to save $1.

Figure 1. In a rotameter, the fluid flow pushes a weighted float up through a tapered tube, which increases the area through which the fluid can pass.

Can alert operator if lines or tool passages become fouled.

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.

Assists energy management.

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 97% efficient motor is diminished if it is coupled to a pump that is operating in the 55% to 60% efficiency range. A flow meter can help ensure that the system operates at its peak-combined efficiency.

Ensures 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.

Figure 2. With a turbine-type flow meter, the rotor blades are magnetized, and the alternating poles pass by a hall effect sensor when the rotor is in motion.

Aids in troubleshooting the entire system.

Often, when a flow problem exists in a process, the user does not know whether the pump or the process is causing it. By separating the two and using a flow meter as a diagnostic device, it is possible to zero in on the problem more readily.

Facilitates taking "snap shots" of thermal capacity throughout the cycle.

If the flow rate 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:

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

Becoming familiar with this kind of information will take some of the guesswork out of future equipment purchases.

Helps recreate the minimum-cycle-time setup for a given tool.

Shops that change tools frequently can easily dial in the flow rate that produced the highest quality parts at the minimum cycle time the last time the tool was used.

Aids in throttling the pump.

Many systems are designed in such a way that the pump should not be run "wide open." Knowing the optimum system flow rate and using a flow meter will help you keep the pump from overloading.

Develop an intuitive know-ledge of flow that can be translated to other areas.

People who often 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 bypass valve makes could come in handy in a situation where no flow meter is available.

Electromagnetic flow meters operate on Faraday's law of magnetic induction, which states that a voltage will be induced in a conductor -- in this case, fluid -- that moves in a magnetic field.

Now that it has been established why the process coolant's flow rate is important, it might be a good idea to examine how it can be measured.

Two things to keep in mind before making any decisions about flow meter selection:

Always choose a meter with a realistic scale. For instance, if the estimated or expected flow rate is 3 gal/min, do not buy a meter that is calibrated for 0 to 300 gal/min.

Most flow meters fall into three broad categories: float-type, turbine-type and electromagnetic.

Float-Type. Also referred to as a rotameter, the float-type flow meter operates on the principle of variable area (Figure 1). 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 a 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, accurate and repeatable.

Turbine-Type. The turbine-type meter is offered in an array of mechanical configurations, but its basic operation is pretty much the same (Figure 2). 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 (because the cross-section of the pipe is known), into velocity and flow rate.

Electromagnetic Flow-Type. This type of flow meter operate on Faraday's law of magnetic induction, which states that a voltage will be induced in 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 converted easily to a flow rate.

A popular saying is "things are seldom as they seem," and this usually applies to process flow rates. Once you get to know your flow, what you find may surprise you. Those surprises could give you important clues about what adjustments to make to squeeze that extra bit of efficiency out of your process.