Companies are aiming to achieve a balance between cost-effective sustainable energy efficiency and ensuring their facilities possess high-quality heating and cooling systems, US technology group Johnson Controls Systems & Building Efficiency Africa GM Neil Cameron tells Engineering News.

It is for this reason, he says, that the Johnson Controls variable air-volume (VAV) controller units are popular worldwide; however, in South Africa, the use of these systems is still in its infancy.

Cameron points out that Johnson Controls System & Building Efficiency has installed several VAV controller units at several US-owned business premises in South Africa in the past two years, including at the American Embassy, in Pretoria.

The company is planning road shows for its existing customers in Johannesburg, Gauteng; Cape Town, Western Cape and Durban, KwaZulu-Natal, as well as in Gaborone, Botswana, to increase local awareness of the system.

“The VAV controller units control and vary the volume of air flowing into a room, reducing the need to pump colder air into a facility while increasing the volume of air,” Cameron explains.

He adds that this process automatically cools the room and requires less energy, as the VAV controller systems raise or lower the output levels of the diffuser, which controls the volume of air pumped into a room.

Pressure Independent

Cameron highlights that the VAV controller unit may be either pressure dependent or pressure independent, owing to the variations in duct static pressure associated with most of the VAV controller systems. However, he notes that pressure-independent controls are preferred.

The accurate volume control achieved by pressure-independent VAV controller units results in substantial energy savings, explains Cameron, adding that it also increases the comfort level for the facility’s occupants.

“Conditioned air is regulated precisely in a room or zone in response to the thermostat demand,” he explains, pointing out that pressure-independent units are equipped with a velocity controller, inlet duct sensor, damper motor and thermostat.

The velocity controller regulates the air-supply volume using the inlet duct sensor to maintain a constant airflow, while the minimum and maximum
airflow requirements are set to suit an area’s applications.

Further, as the air-conditioning load in a particular space changes, Cameron says the thermostat signal resets the velocity controller to modify the air supply to suit the requirements of that space.

“At any given setting, the controller will maintain constant airflow, regardless of changes in inlet static pressure,” he notes.

System Benefits

Cameron says the advantages of the VAV system for property owners include savings in energy costs, as accurate measurements of the air required to achieve occupant comfort are supplied. Simple installation for either new or retrofit projects, simple revision and the resetting of maximum and minimum air quantities to suit changed zone loads can be done in a matter of minutes using the thermostat.

Owing to this, it is no longer necessary to inconvenience tenants by having to access the ceiling space to reset air quantities, he adds.

Further, property owners can use their own maintenance staff to monitor and maintain the VAV controller units.

“The system provides local and remote monitoring, as well as control, through the company’s building management system, which enables maintenance staff to read zone temperatures and actual airflow from a central control station and reset control parameters without entering a tenant’s premises,” he explains.

Additionally, in most commercial buildings, the architect has to design according to a modular grid and the design plan layouts have to be flexible enough so that changes can be made, since the architect rarely knows what tenancies and portioning will be required at the design stage.

Cameron points out that the VAV controller units enable an architect to reset and recalibrate the maximum and minimum air quantities. This allows for the partitioning of a space to suit the needs of the tenant, as air can be moved around in the building to provide the correct air quantities for each location.

The reset capability of the VAV unit’s thermostat enables such shifts to be made without accessing the ceiling, which eliminates the risk of workers damaging or soiling ceiling tiles, he says.

Diagnostics

Cameron highlights that Johnson Controls Building Efficiency’s VAV controller units are equipped with a comprehensive set of diagnostic functions.

“The system’s diagnostic functions ensure that operators are aware of any problems before the occupants of a building are,” he emphasises, adding that diagnostics enable the operator to manage the problems, rather than react to them.

Cameron notes that the controller unit is continuously performing diagnostics of its electronic components and, if any of the analogue inputs fail, the controller can detect the failure, report the problem to the units’ building automation system and use a default value instead.

“Usually when power is restored after a power failure, all the units in a building open to provide cooling and if all the actuators engage at once, this may cause a demand peak. To avoid this, each controller has a delay timer to ensure loads can be staggered after a power failure,” he highlights, adding this is of particular importance in South Africa, where regular power outages have occurred in the past five years.

Subsequently, the controller keeps track of the actuator run time and the controller time. The average duty cycle of the actuator can be calculated, as a high-duty cycle can indicate that the control loop is cycling and requires attention.

“If the output to the actuator is at 100% for more than the minimum time required for operation, this will set a ‘starved box’ flag, indicating that the system static pressure may be too low or the fan may be undersized,” notes Cameron.

The controller also keeps track of the average difference between space temperature and the setpoint, as well as of the average difference between actual flow and the desired flow setpoint. A high value for either of these diagnostic statistics may indicate that the controls are recycling or not working, states Cameron.

Damper Actuator

Johnson Controls Building Efficiency engineers were given the mandate in 2011 to develop the quietest VAV damper actuator in the industry, Cameron says.

“After many tests, they have succeeded in limiting the noise level to less than 35 dB, with the actuators having more than 100 000 full stroke cycles and two-million positions,” he notes.

Moreover, he points out that the actuators used with the VAV controller units are an incremental type, which can be positioned anywhere between 0° and 90° by the controller unit.

“The controller keeps track of the actual position of the damper and calibrates itself every 24 hours,” says Cameron.

Differential Pressure Transducer

The differential pressure transducer is connected to the multipoints velocity sensor at the inlet of the VAV controller, explains Cameron, adding that one of the pickup elements of a velocity sensor has holes facing the airflow, enabling it to sense the total pressure.

He notes that the other pickup element has a hole facing away from the airflow, which senses static pressure.

“The difference between total pressure and static pressure is velocity pressure. From the velocity pressure, the controller calculates the airflow passing through the box,” Cameron explains, adding that the accuracy of the differential pressure transducer is important, as it calculates airflow.

“The controller will automatically calibrate the differential pressure every 24 hours to maintain accuracy,” he concludes.