Terminology is the most confusing part of our control industry.

 EQUAL PERCENTAGE?

 LINEAR?

· LOGARITHMIC?

· ANTI LOG?·

 QUICK OPENING?

Confused? Welcome to the club!

Terminology is the most confusing part of our control industry. Equal percentage does not mean equal stem movement relative to signal input. It means equal percentage of heat output relative to signal input. Essentially therefore, equal percentage characteristic is logarithmic, quick opening is anti-logarithmic, while Linear means equal percentage of water flow relative to signal input. So why is equal percentage not equal to linear? Because cutting the flow to a heating coil by half only reduces the heat output by 10%. Consider this coil:

At design with full flow
Entering water temperature    Leaving water temperature    Mean water temperature    Flow
180° Fahrenheit                            160° Fahrenheit                       170° Fahrenheit                    1 gal.
(The mean temperature of the coil is 100 Fahrenheit degrees above room temperature.)

At 50% flow
Entering water temperature    Leaving water temperature    Mean water temperature    Flow
180° Fahrenheit                            142° Fahrenheit                        161° Fahrenheit                    ½ gal.
(The mean temperature of the coil is 91 Fahrenheit degrees above room temperature.)
(The coil still generates 91% of the heating effect with but half the water flow.)

What happened is that the water hung around in the coil a while, but continued to loose its heat. There are numerous after market consultants wise to this effect, who make a good living just visiting colleges, hospitals and universities asking for 50% of the energy savings they can find. Once the contract is signed, they pull out a load of 7 HP 1750 RPM pump motors, replace with 1 HP 900 RPM motors, returning a year later for the payback. Because they know that HP costs as the cube of the speed, they're saving 6 HP or 4 ½ kW per hour, 8 or 12 months of the year, and they can't shortchange the system or overwork the motors. Essentially, they go back and size the pump correctly! They save the owners about $ 2500 per pump per year, so they go back and claim their half a year later per pump!

Valve manufacturers also know this ploy, so they design equal percentage characteristics into their control valves. To get a reduction of 50% of the heating output, the valve cuts back to about 10% of the flow and the characteristics look like this:

At 10% flow
Entering water temperature    Leaving water temperature    Mean water temperature    Flow
180° Fahrenheit                            say 70° Fahrenheit                   120° Fahrenheit                      0.1 gal.
(The mean temperature of the coil is 50 Fahrenheit degrees above room temperature.)
(The coil still generates 50% of the heating effect with less than 10% of the flow.)
(The coil or radiator is hot only at one end.)

Unfortunately, equal percentage doesn't work well for cooling. Cooling is best done with the face of the coil enough so that adequate dehumidification occurs, and therefore, full flow is required. And full flow is required in a mixing application, as in an indoor-outdoor control application, where boiler water and return water are mixed to supply tempered water for mild days. In these applications, having equal percentage plugs in the ports of a three way valve causes starvation in the mid point since each port is then theoretically reduced to 10% of flow or total 20% flow. All right for a mixing valve in diverting application, but not good for a mixing valve or cooling valve.

But Spartan Peripheral Devices needed one valve suitable for all duties, heating, cooling, mixing three way or just two way. The design had to involve excellent modulation without mid range starvation in three way mode and equal percentage in two way as well. It also had to involve greatest operational efficiency so that smaller more compact actuators could do the work of more bulky motors. And finally, in three-way applications, heat or cooling, diverting applications or mixing, it had to allow for full flow in the mid position so as not to starve cooling systems, or limit the flow of mixed indoor-outdoor system water supplies, yet meter small flows efficiently for good modulating control. A tall order. Spartans unique approach to this problem was twofold. First, the parabolic and slotted skirts of the plugs were limited to initial travels only thereafter being linear to valve stem position. And second, in order to obtain the balance of the equal percentage characteristic, the linkage was provided with a geneva type movement, which provided a smooth transition from equal percentage to mid range linear flow, and without starvation of total flow.

A by-product of this design was the effect on close-off pressures. Because the geneva movement gave greater gearing ratio at the ends of its travel; it also provided more stem force with a smaller actuator. It became a three-way winner! Better modulation, little or no mid range starvation, greater close-off force, smaller actuators and a compact (less than 7" high) assembly on valves up to 4".

A sideline, but interesting detail is the port finder. Numerous three-way control valves, inadvertently installed backwards, cannot be truly diagnosed, because the insulation or lagging hides all; port lettering included. Spartan has cast a ridge at the top of their yoke, pointing to the 'A' port. Without removing the insulation, a little probing at the top of the valve soon locates it. Tapping points at all ports also make these robust valves suitable for many process applications.

D.S. McIntosh - President of Spartan Peripheral Devices, Montreal, Canada
www.spartan-pd.com

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