Throttled Into Submission
What do you get when you install a 1,000-horsepower engine in a Volkswagen Beetle? Answer: A vehicle that probably could cruise along at 70 mph with the gas pedal only pressed down a quarter of an inch!
Imagine what it would be like driving such a car. Those of you with a steady right foot could probably keep it moving somewhere between 60 and 80 mph on the freeway. But even NASCAR's Jeff Gordon might look a bit erratic trying to drive this little hot rod around in city traffic.
The problem with this scenario is that the driver is forced to use a tiny fraction of the gas pedal's movement to regulate the car's speed over its entire useful range. (Unless, of course, he decides to challenge the world's land speed record.)
As far-fetched as it sounds, lots of variable speed pump injection-mixing systems have a similar "rangeability" problem. The injection pumps in these systems only reach a fraction of their full speed, even while supplying heat at design load conditions. While many of these systems produce a comfort level that's apparently above the callback threshold of their owners, the ability of their injection controls to fine-tune water temperature is largely wasted.
Underrevving Pump SyndromeTwo factors often prevent a variable speed injection pump from operating above the lower portion of its speed range, an affliction I call underrevving pump syndrome (UPS).
1. A relatively low injection flow rate, which is needed by many low temperature radiant systems supplied from conventional boilers.
2. An infinitesimally small head loss, which is associated with the short injection riser piping used in most systems.
UPS is a relatively new ailment within the hydronics industry, first appearing as variable speed mixing systems gained a foothold in the market a few years ago. As the popularity of variable speed injection mixing has grown quickly, so have the instances of UPS.
Fortunately, UPS is fully curable. First we'll examine the causes and then discuss the antidote.
The injection flow rate needed to supply a given heating load can be determined using Formula 1:
fi = the required injection flow rate (in gpm)
Q = the load being supplied (in Btu/hr.)
b = a constant (use 490 for water, 470 for 30 percent glycol, 450 for 50 percent glycol)
Thot = temperature of ingoing injection water (in degrees F)
Treturn = temperature of cool fluid returning from distribution system (in degrees F)
It's often amazing just how small the hot water injection flow rate needs to be. For example, say we want to supply a design heating load 80,000 Btu/hr. to a slab-type floor heating system that has a return water temperature of 100 degrees F. Assume boiler water is available at 180 degrees F. The required injection flow rate is attained using Formula 1:
The filter pump in your kid's aquarium could probably keep up with this flow requirement.
The injection piping usually consists of 4-8 feet of 3/4-inch copper tubing coupled the boiler loop and distribution loop with pairs of closely-spaced tees as shown in Figure 1. The closely spaced tees uncouple the head loss of the injection riser piping from those of the boiler loop and distribution loop. The injection pump only operates against the head loss of the injection risers.
So what's the head loss of, say, 6 feet of 3/4-inch copper tube at a flow rate of 2 gpm? According to my calculations, it's about 0.06 feet of head. A requirement so low that even a 1/40-horsepower circulator is as much overkill as a 1,000-horsepower engine in a Volkswagen. You can see this in Figure 2, where the desired operating point of 2 gpm and 0.06 feet is plotted along with the pump curve of a 1/40-horsepower wet rotor circulator.
The system resistance curve for the assumed injection piping (6 feet of 3/4-inch copper) is shown in blue. This curve intersects the pump curve at a flow rate of about 9.3 gpm. If you install this pump into the assumed injection piping and allow it to run at 100 percent speed, 9.3 gpm is the injection flow rate you'll get - more than four times higher than necessary under design load conditions.
So What?Maybe you're thinking, "What's the big deal if the injection pump never needs to run more than 10 percent or 15 percent speed? Just think of all the reserve capacity that pump has and all the electricity I'm saving. Besides, since I usually size my boilers two or three times larger than necessary, why not do the same for the injection pump?" (If you identify with these statements, I'll bet you also think a 1,000-horsepower Volkswagen would look pretty cool on car night down at the drive-in.)
Unfortunately, injection controls can't "drive" heating systems with improperly setup injection pumps any better than you could drive that souped-up VW. Here are some problems that will occur:
Problem No.1: The ability of the injection control to "fine-tune" heat input using an underrevving injection pump is severely handicapped.
Imagine driving your car if the gas pedal could only be set at three or four fixed positions. You could probably get by, but the ride's certainly going to be a bit jerky.
Similarly, at the lower end of their speed range, most injection controls adjust pump speed in steps rather than as a continuous process. The more restricted the pump speed range, the fewer speed steps the control has to work with. Although higher mass radiant systems can partially mask this deficiency, operating a system in this manner wastes much of the resolution the injection control is otherwise capable of.
Problem No. 2: An injection control matched with an underrevving pump repeatedly crosses the speed thresholds where the boiler is turned on and off.
Although most injection controls are programmed to keep the boiler on for some minimum time once fired, that time may be shorter than what could otherwise be attained if the boiler remained enabled, and was set up with a wider operating differential. The latter allows the thermal mass of the boiler to be "exercised" to generate longer, more efficient burner cycles. (See last month's column.) An injection control that repeatedly turns the boiler on and off is usually suffering from UPS.
Spin Those Impellers!One way to cure UPS is to use an injection pump that's only capable of a few gpm with a head gain of, say, 0.5 feet.
Go pull all your pump catalogs off the shelf and see if you can find one with these specs that's also capable of lasting a couple of decades in a typical hydronic system. If you do, it probably costs quite a bit more than a garden-variety, cast-iron zone circulator. Some pump manufacturers are beginning to address this need with low flow/low head/low cost models specifically designed for injection-mixing applications. Thanks, pump guys. This is definitely a step in the right direction.
When a typical zone circulator is used as the injection pump, the cure for UPS is to add flow resistance to the injection riser piping. A throttling valve in the return injection riser does the trick. Think of this valve as a sort of rheostat for water. The more it's closed, the greater its resistance to flow.
The throttling valve should be closed until the injection pump is running at full speed while delivering the design injection flow rate calculated from Formula 1.
Envision the blue system resistance curve in Figure 2 as getting progressively steeper as the valve handle is turned toward its closed position. When the system curve is steep enough to pass through the pump curve at the design injection flow rate, the valve is set properly.
If we make a few assumptions about the injection riser piping, it's possible to calculate the Cv of the throttling valve necessary to force the injection pump to the design load's full speed.
Cv = the required Cv setting of the throttling valve
fi = the calculated injection flow rate from Formula 1 (in gpm)
Hp = the head of the injection pump at the injection flow rate fi (in ft.)
r = 0.058 when the injection riser are 3/4-inch copper
r = 0.25 when the injection risers are 1/2-inch copper
This formula assumes that the total equivalent length of the injection risers, excluding the throttling valve, is about eight feet. Be sure to use the correct (r) value, depending on the pipe size of the injection risers.
For example: To achieve a full speed flow rate of 2 gpm through 3/4-inch injection risers using the 1/40-horsepower circulator of Figure 2, the throttling valve should have a Cv of:
The Cv rating of a valve is the flow rate (in gpm) of water at 60 degrees F, which produces a pressure drop of 1 psi across the valve in its fully open position. At any partially open position the valve has a unique Cv value. The more the valve is closed, the smaller its Cv value becomes.
Some balancing valves have a Cv indicator built into their handles. If that's what you're using, just make the calculation using Formula 2, set the handle to the indicated Cv, and you're done.
Another option is to install a flow meter, or a precision-balancing valve with flow metering ports, into the return injection riser piping. Close the valve until the flow rate calculated with Formula 1 is attained, and you're done.
So what should you do if you don't have a valve with a Cv scale or a flow meter? In a future column I'll show you a way to use temperature readings to set the throttling valve on an injection-mixing system. This method is especially helpful during system start-up, when the water temperature returning from the floor slab can be quite a bit lower than under normal operating conditions.
In the meantime, don't be afraid to choke down the throttling valves on your variable speed injection systems. Your injection control will return the favor by driving the system with the same deft precision as a NASCAR champion.