|
February 2011
Column
AutomatedBuildings.com
|
[an error occurred while processing this directive]
(Click
Message to Learn More)
|
|
Controls Devices (Part 1 of 3)
Rules of thumb to follow for TC installation & design
|
Steven R. Calabrese
Control Engineering Corp.
Contributing Editor
|
This month we get back to basics with a column about modern temperature
controls devices. Note that the term “temperature controls” is a
catchall for all controls devices, not just thermostats and temperature
sensors, but pressure and humidity sensors, current-sensing switches,
safety devices, and end devices. In the DDC domain, all of these
devices, in one way or another, wire back to a digital controller,
which in turn monitors inputs and affects outputs in conformance with
the intended Sequence of Operation. We’ll not talk about the controller
this time around, just about the stuff that connects to the controller,
specifically the topics in the order that follows:
- Sensors & Transmitters
- Switches & Two-state Devices
- Safeties & Limits
- End Devices
This installment is part one of a three-part series. When completed in
April, the series can serve as a good “quick reference” guide to
temperature controls devices, covering not all but a good amount of
what makes up a typical HVAC temperature control system in this day and
age.
Sensors & Transmitters
OAT Sensors
Outside air temperature sensors need to be located outdoors (yeah!),
preferably on a northern exposure, and out of the direct sunlight. Keep
away from areas where rainwater may accumulate. And while you’re at
it, go the extra mile and get the temperature + humidity variety. Yeah,
it’s more money, but monitoring outside air humidity is probably
specified anyway, and if not, you really do get bang for your buck
on spending the extra dough. Labor-wise it’s an extra pair of wires, so
no real additional cost there. And controller-wise, you eat up an
input, so unless you are down to your last input and would need to add
another controller to accommodate the humidity sensor, you really
should “default” on providing it.
Duct & Pipe Temperature Sensors
Probes are your “general purpose” duct temperature sensors. They come
in a multitude of lengths, and there are various rules of thumb for
properly selecting them. One states that the probe should reach halfway
into the duct, which probably makes good sense, as far as getting a
good, consistent temperature reading, as that is the point at which the
velocity of the air is at its greatest, with the velocity dropping off
as you reach the sides of the duct. Given this rule of thumb, it
follows that, for terminal units (VAV and fan-powered boxes), a duct
probe of 8” is a good all-around standard size.
For larger ducts (over 4’ wide), use an averaging sensor, which is not
a probe but a continuous pliable metal tube with temperature sensors
embedded within, along the length of the tube. A good rule of thumb
dictates that, for every square foot of cross-sectional area of duct,
provide one linear foot of averaging sensor. Also use an averaging
sensor where temperature stratification is likely to occur, as in the
mixed air chamber of an air handling unit.
Pipe temperature sensors generally come in two varieties: immersion and
strap-on. Immersion sensors are for new construction, to be installed
in the pipeline prior to completion and system fill. Standard sizes
suitable for most HVAC-level applications are 2” and 4”. Installation
consists of welding the sensor “well” into the pipe, and then screwing
the sensor into the well. If you have a pipe size of less than 2”, use
a strap-on sensor, along with some thermal conductivity grease.
Strap-on sensors are also good for existing operational systems, and
other “specialty” situations (like in new construction when the sensor
is overlooked or all but forgotten about until the very end of the
project after the system has been filled!).
Space Temperature Sensors
Space sensors come in a variety of styles. It takes some up-front
thought before deciding on a particular style and specific options.
Does the application call for a decorative enclosure or low-profile
one? Is visual indication of room temperature and setpoint important?
What about occupant adjustability? An override pushbutton, to allow
override to the occupied mode during “off” hours? Answers to these
questions require some insight as to how the end-user will use their
system. Sometimes visual indication of the temperature and setpoint
does more harm than good. And occupant adjustability may be deemed to
be undesirable, although with the typical DDC system you can limit the
adjustability of the temperature range of the dial or slidebar, pretty
much down to 0 degrees.
For public areas, such as corridors, and for areas of mass
congregation, such as classrooms and meeting halls, a tamper-proof
plastic guard may be required, even if the sensor has no amenities.
Metal cage guards are necessary for gymnasiums, where a stray
basketball could otherwise eradicate a wall-mounted temperature sensor.
[an error occurred while processing this directive]Although nowadays, “vertical” mounting is the norm for the kinds of
space temperature sensors that most BAS accommodate, in retrofit
applications, you may run across a situation in which you need to
replace an old horizontal-mount thermostat with a new vertical-mount
temperature sensor. Wall plates specifically manufactured for these
applications (sometimes called “goofplates”, presumably named so after
an installer cut in an electrical wall box in the wrong orientation and
thus had to use one of these) are available which cover the existing
hole in the wall and provide mounting holes for the new sensor.
Humidity & CO2 Transmitters
As discussed above, an outside air humidity sensor makes good sense
(pun intended!), and is necessary in order to make outside air enthalpy
calculations, which are required for economizer changeover decisions
based on both outside air temperature and humidity. For instance, when
it’s warm yet dry outside, there may be an opportunity to economize. On
the other hand, if it’s cool and rainy, you may not want to bring in
outside air if it’s going to contribute to an indoor humidity problem.
An enthalpy calculation is required to allow the BAS to make these
types of decisions. As far as return air goes, RA humidity is a good
indicator of the average space relative humidity levels, and is a
typical spec item for larger air handling systems.
Carbon dioxide (CO2) sensing has its place in HVAC control, nowadays
more than ever! As CO2 is a good indicator of human occupancy, CO2
transmitters are used in indoor environmental control strategies, in an
effort to maintain a suitable “fresh-air” environment within the
occupied building spaces. An outside air CO2 transmitter can serve as a
reference for Demand Controlled Ventilation (DCV) strategies. The
outdoor air CO2 level is measurable and while generally consistent, it
can vary due to certain environmental conditions. By comparing indoor
levels with the outdoor level, an effective DCV strategy can be
implemented. Return air CO2 level is a good indicator of the average
levels of CO2 within the occupied spaces, however for “true” DCV,
individual space CO2 transmitters may be required, especially when
there’s occupant diversity within the building, as is true with
schools, theatre houses, and even commercial office buildings.
Tip of the Month: Tech-talk…what’s the difference between a sensor and
a transmitter? From a functionality standpoint, not much. Which is why
these terms are often used interchangeably. Technically speaking, a
sensor is a passive device requiring no separate power, as with a
common thermistor type temperature sensor. Hook a pair of wires up to
it and read the resistance across it, which is a function of the
temperature surrounding it, and you have your temperature value. For
sensing humidity and pressure, active electronics are typically
required, and so these sensing devices are called transmitters. In our
everyday vernacular, most of us tend to refer to any device that senses
something as a sensor, even when it’s technically a transmitter. In the
end, as long as we understand the difference, we should be “good to
go”.
footer
[an error occurred while processing this directive]
[Click Banner To Learn More]
[Home Page] [The
Automator] [About] [Subscribe
] [Contact
Us]