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Controls Devices (3 of 3)
Rules of thumb to follow for TC installation & design
Steven R. Calabrese
Finally we reach part three of this three-part series on control
devices. As I did with the first two parts to this series, I do here as
well, listing the topics of discussion in the order that they’re being
presented throughout this series:
This final installment will discuss the last two topics in the above list, and thus closes out this series on temperature controls devices.
Access to the previous installments (Part 1) & (Part 2)
Safeties & Limits
Types & Functionality
Low Limit Temperature Controllers, or Freezestats, as they are more commonly referred to, consist of a vapor-filled capillary, typically 20 feet in length, but can be shorter or longer (depending on the manufacturer). The capillary gets draped across the hot or chilled water coil, the rule of thumb stating that for every 1 square foot area of coil, there should be 1 lineal foot of capillary. For coils larger than 20 square feet or so, this means that more than one freezestat is required to do the job, which is to prevent the coil from freeze-up conditions.
High Limit Discharge Static Pressure Switches are dual-port devices, of which the high side gets ported into the discharge air duct of an air handling unit. The purpose of the device is to prevent excessive pressure from building up in the supply duct, typically of a VAV air handler.
Low Limit Suction Static Pressure Switches are essentially the same devices as the high limits, except that it’s the low side that gets ported into the duct. The device is meant to prevent the “negative pressure” chamber of an air handling unit from becoming excessively negatively pressurized. As such, the low side of this device will typically be ported into the suction side of the supply fan.
Duct Smoke Detectors sample the air in the duct, drawing in air via one port and expelling it back into the airstream via the other. The purpose of this safety device, of course, is to detect smoke in the airstream, and prevent further circulation of it once detected.
Common to all safety devices is that they are of the “manual reset” variety, meaning that when they “trip”, they stay tripped until they are manually reset by some form of pushbutton or other resetting mechanism integral to the device. The setpoint adjustment of these devices is essentially a “trip” point adjustment. When the trip point is surpassed, either upward as is the case with the high limit static pressure switch, or downward as is the case with the freezestat, the device trips out, and its electrical contacts are opened, thus breaking the safety circuit.
These devices are typically single-pole devices, meaning that they have only one set of contacts, or one switch. It is common practice to have each safety device control, with its single contact, a separate double-pole double-throw (DPDT) relay. Then one contact of the relay is utilized for the “hardwired” shutdown of the associated equipment, and the other contact is used to report status to the Building Automation System (BAS). Often, the hardwired shutdown contacts of the relays of the various safety devices serving a given system or piece of equipment are wired “in series”, such that a trip of any device, will, for all practical purposes, break the chain and invoke a “failsafe mode”.
When a safety device trips, certain things must happen, and they need to be incorporated into the design of the control system. In a typical air handling unit, the following are what’s to take place, presuming that all control valves and dampers are equipped with “spring-return” actuation:
For constant volume fan systems, the fans are controlled by motor
starters. The safety devices need to be wired into the Hand-Off-Auto
switches such that they affect a shutdown in either the Hand or the
Auto mode. For variable volume fan systems, when the fans are
controlled by variable frequency drives, the safeties need to be wired
into both the “drive” and the “bypass” modes of the variable frequency
The popular abbreviation for these, as found on mechanical plans and shop drawings, is MOD, or motor operated damper. The MOD is the complete package: damper plus actuator. However the damper comes first, and then the actuator(s), in terms of sizing and selection.
For damper sizing, it is generally accepted that, for two-position control, the damper can be “duct size”. For proportional control applications, the consultant may call for proper damper sizing based on pressure drop, however in practice, this often gets sidelined in favor of “full size” dampers.
For large damper applications, multi-sectional dampers are required, in that each damper section can be operated individually by its own damper motor. The maximum square foot area per section varies from spec to spec, however generally speaking, the sections need to be “small enough” to be actuated comfortably by the damper motor. A good rule of thumb is that no damper section shall exceed 15 square feet.
Once popular but now fallen from grace, the globe valve was for generations the traditional control valve of choice. Why the decline in popularity? Because of the “genetically altered” ball valve! More on this in a bit. The globe valve is still a viable choice for control in applications ranging from 2” to 6”, especially in the higher end of that range, where ball valves have yet to invade and conquer. And in steam control applications, the globe valve is still king, I suppose. Yet for hot and chilled water applications calling for smaller valve sizes, there are cheaper (less costly) and perhaps more efficient options that have come to market in the past decade or two.
At some point in time, someone decided to stick an actuator on a standard quarter-turn ball valve. And the industry was turned on its head! Seriously though, it took more than that to bolster the popularity of the ball valve. These valves are, in basic form, not very linear, so there needed to be some manufacturing engineering to take place to make them operate in a more linear manner. Now that that has taken place, the ball valve has taken its seat next to (or above) the globe valve in terms of popularity, at least in the ½” to 3” range or so.
Butterfly valves take over where globe and ball valves leave off. Not much to say here, other than the cutoff for globe valves is (in the practical context, anyway) 6”. So for control valve applications requiring valves greater than 6 inches, the butterfly valve is the preferred way to go.
Variable Frequency Drives (VFDs)
VFDs are categorized here as end devices in that, like with motorized dampers and control valves, they receive a signal by which they respond to in a certain way. A VFD in fact needs to receive two signals to do its job. One is a simple start/stop command that tells the VFD to start turning the motor. The other is the speed reference signal, which tells the VFD how fast to turn the motor.
|Tip of the Month: Don’t skimp on the safeties! Don’t compromise the control system design of mechanical equipment by shortcutting on the required safety devices. As the saying goes, an ounce of prevention…anyway, the point is, there is simply no good reason for not giving the proper attention to this aspect of control system design. If the air handling system is capable of generating excessive pressures in the duct system, install the static pressure high limit. Likewise, if the chilled water coil is larger than 20 square feet, install another freezestat. The consequences of poorly protected mechanical equipment far outweigh the cost savings of not putting these all-important devices in place!
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