Sunday, November 1, 2009

HVAC/R Electronics # 6

by Roger Desrosiers

Improving air quality is another opportunity for engineers to apply the capabilities of digital systems. Energy conservation strategies require buildings to be airtight to prevent the escape of expensive heated or cooled air. This usually comes at the expense of ventilation. “Sick building syndrome” has become a national concern as researchers continue to uncover the effects of indoor air quality (IAQ) on human health and productivity in the workplace. Concerns over health hazards in the workplace and the spread of airborne contaminants are issues that have reached the forefront of public attention. The control of building ventilation is a problem that is being solved through the application of digital controls. So let’s take a look at a ventilation program.

Ventilating Control Program



Sequence of Operation:



1. Supply fan starts and enables return fan start and system controls.
2. SA smoke detector stops supply fan when smoke detected.
3. RA smoke detector stops supply fan when smoke detected.
4. Controller stops fan when low temperature detected.
5. SA high static pressure control stops fan when unsafe pressure exists.
6. Automatic fan system control subject to commandable on-off-auto software
point.
7. Control program turns supply, return, and exhaust fan on and off dependent
upon optimized time schedule, unoccupied space temperatures, and occupant
override requests.
8. Occupant override switch provides after hours operation when pressed.
9. Duration of operation for override request.
10. Space temperature (perimeter zone) inputs to optimum start-stop, unoccupied
purge, and low limit programs.
11. Set-point at which unoccupied low-limit program executes.
12. OA temperature input to optimum start-stop program.
13. Return fan operation enables exhaust fan control program.
14. Exhaust fan status (operator information).
15. Warm-up mode status (operator information).
16. Supply fan load (VAV type systems-operator information).
17. Return fan load (VAV type systems-operator information).

Air handling system shall be under program control, subject to supply air (SA) and return air (RA) smoke detectors, SA high pressure cut-out, and heating coil leaving air low-temperature limit control; and shall be subject to system software on-off-auto function.

Supply fan shall be started and stopped by an optimum start-stop seven-day time schedule program, an unoccupied low space temperature limit program, or by an occupant via push button request. The push button shall be integral with the space temperature sensor. Any push button request shall provide sixty minutes (operator adjustable) of full system operation. Return fan shall operate anytime the supply fan proves flow (via a current sensing relay). The exhaust fan shall operate during scheduled occupancy periods and during occupant requested after-hour periods anytime the return fan proves flow.

In figure 1 we see a supply air control loop with the sequence of operation.



Sequence of Operation:

The DDC controller uses a temperature sensor mounted in the supply air duct to modulate control valves or mixing dampers to maintain a supply air temperature set-point. In most systems that employ a heating and cooling coil, the hot water valve and the chilled water valve should be modulated in sequence.

When the supply air temperature falls below set-point, the hot water valve begins to modulate open and consequently, the cooling valve begins to modulate closed. If the supply air temperature continues to fall below the set-point, the heating valve will open fully and the cooling valve will close completely.

When the supply air temperature rises above set-point, the hot water valve begins to modulate closed and consequently, the cooling valve begins to modulate open. If the supply air temperature continues to rise above the set-point, the heating valve will fully close and the cooling valve will open completely.

A temperature sensor located in the mixed air stream (between the unit filters and the coils) is used to provide mixed air low limit control. When the temperature sensed by this element falls below the setpoint, the outside air damper fully closes, the return air damper fully opens, the exhaust air damper closes to a minimum position, and the valves on all coils will fully open. This sequence should always be used on systems with wetted coils.

When the unit fan is turned off, the outside air damper fully closes, the return air damper fully opens, the exhaust air damper fully closes, and all control valves return to their “normal” positions.

Design Considerations:

Control Valves:

· Avoid using spring return actuators on control valves for wetted coil
applications.

· When selecting two-way valves for control
of wetted coils:

· For hot water coils, have the valve configured in the “normally” open position.

· For chilled water coils, have the valve configured in the “normally” closed
position.

When selecting three-way valves for control of wetted coils:

· For hot water coils, have the valve piped such that when the valve is in the
“normal” position, the water flows through the coil.

· For chilled water coils, have the valve piped such that when the valve is
in the “normal” position, the water bypasses the coil.It is recommended that
mixing valves be used in all three-way applications unless otherwise
specified.Also be cautious to not pipe globe valves that are designed
for mixing applications for diverting service. The fluid flow will
cause a“hammering”effect and severe noise and damage will follow.

In Electronic #5 I discussed how to program a control loop, following are some of the most common loops that can be controlled that you will find on sophisticated controls.

1. Discharge Air Temperature
2. Mixed Air Temperature
3. Hot Cold Deck Temperature
4. Cold Deck Temperature
5. Humidity or Dew Point Control
6. Indoor Air Quality Control
7. Ventilation Control
8. Supply Fan Static Pressure Control
9. Supply Fan Start/Stop Control
10. Return Fan Start/Stop Control

One of the last steps is to connect the analog inputs and outputs to an 8x computrol controller as shown in figure 1 below.



From this diagram you can see that the DDC is powered with 24 volts supply at the terminal strip in the lower left side. The discrete (on-off) inputs are called binary inputs. They are connected to any terminal that you designate as a binary, because you have the option to designate any of the terminals you like, which is a feature of these controls. Therefore point 1 thru 8 can be an analog input or output or it can be a binary input or output.

One of the main advantages of a DDC system is that it can be connected to a network and be controlled remotely. In some cases the control is setup in a special room, where the HVAC technicians for a large building or campus can monitor the entire system from a single console. Another feature of the network is that its data can be transmitted over dedicated telephone lines or connected through the Internet so that the data are available worldwide. Other features such as energy management, security, fire control and other essential functions can be controlled over these networks.

Conclusion:

This brings me to the end of this series of discussions on the wonderful world of DDC Controls and some of the attributes and marvelous things that can be done with these controls. My intention was not to teach you all about these controls but more of a fundamental introduction. If I aroused your interest into looking further into how these controls work then I will feel that I have done some good for the betterment of this industry. If you are interested in learning more about DDC Controls I urge you to peruse the following web sites:
DDC-Online
Computrols.com

copyright(c)2009
Roger J. Desrosiers



About the Author: Roger is a contributing faculty member of HVACReducation.net He has over 40 years experience in Air Conditioning and Refrigeration. He is also a member of R.S.E.S., CM, The Association of Energy Engineers, Certified Energy Manager, ASHRAE, Certified Pipe Fitter United Association and is 608 Universal Certified.Lean More About Roger!