Sensing Options | rickardair

Sensing Options

Room temperature sensing is one of the most crucial parts of the VAV diffuser since the basis of any control strategy is the difference between required room temperature (setpoint) and current room temperature.

Room temperature sensing is one of the most crucial parts of the VAV diffuser since the basis of any control strategy is the difference between required room temperature (setpoint) and current room temperature.

The location of the room sensor can have a dramatic impact on the accuracy of sensing, and as a result the control action to achieve the required room temperature. If the room temperature is sensed incorrectly, the controller can believe the room is in control, even though it could be far of required temperature.

OPTIONS:

Wall thermostat / controller / set point adjuster.

On-board Controller with on-board sensor with or without infrared adjuster.

On-board controller with remote sensor with or without infrared adjuster.

WALL THERMOSTAT / CONTROLLER / SET POINT ADJUSTER

The modern looking Wall thermostat and adjuster, sense space temperature on the wall, at about shoulder height. The benefit is that the sensor is in the occupied zone of the space and as such should measure temperature in the occupied space.

Consider the following:

The wall temperature could influence the sensor if the wall temperature is substantially different to the space temperature. The proximity of heat generating equipment could influence the sensor.

The proximity of a cold or hot window façade could influence the sensor.

Ensure that the sun do not shine on the wall thermostat, influencing the sensor.

Ensure the wall thermostat is located in the occupied space. This is especially important if the room is has a window façade. 

If the room is large and have distinct different zones with more than one diffuser, more than one wall thermostat should be considered. 

ON-BOARD CONTROLLER WITH ON-BOARD SENSOR

In many modern buildings set point is controller via BMS software. The end result is that in many applications Wall Thermostats are not used. Rickard diffusers supply air at high velocity at ceiling level while maintaining cocianda. The air then drops down into the occupied space at the walls or when the throw limit is reached.  The end result is that air is induced from the room flowing towards the centre of the diffuser. The quantity of induced air is maintained even at minimum air due to the exit Geometry being changed by the control disc maintaining high exit velocities. The center of the diffuser is therefore the ideal position to sense space room temperature. Rickard further enhanced the accuracy of sensing by designing an low pressure induction system that actually induce  room air onto the sensor.

Consider the following:

If supply air is within 10 degree of room temperature accuracy of sensing is within one degree of the temperature 100mm below the diffuser trim plate for VCD, BSD style diffusers and within 1.5 degrees of the temperature 100 mm below the diffuser trim for the VSW and VLN style diffusers.

During heating stratification can become a big problem. If the diffuser neck pressure is above 30Pa (maintaining high exit velocities) and if supply temperature is limited to 10 degrees above room temperature, offsets as per above applies. If not it can be expected that the sensing offset will be higher. 

ON-BOARD CONTROLLER WITH REMOTE SENSOR

In many modern buildings set point is controller via BMS software. The end result is that in many applications Wall Thermostats are not used. If it is expected that the supply temperature will be more than 10 degrees Celsius above or below room temperature or when stratification is expected in winter, Rickard suggest the use of remote sensors. These sensors can be fitted in position that is representative of conditions in the occupied space.

Consider the following:

Make sure the position of the sensor is in the occupied zone.

Make sure there is no influence from the mounting surface (wall) that could be warmer or colder than the zone.

Make sure there is no load close to the sensor that could influence the accuracy 

MLM SOFTWARE TEMPERATURE COMPENSATION:

In many modern buildings set point is controller via BMS software. The end result is that in many applications Wall Thermostats are not used.

If it is expected that the supply temperature will be more than 10 degrees Celsius above or below room temperature or when stratification is expected in winter, the temperature reading on the diffuser will be influenced by local conditions under the diffuser in will differ substantially from the occupied space. Rickard provides a system in the MLM software to compensate for these local effects.

Software Compensation for incorrect sensing:

A supply air sensor is used to determine degree off back end heating or cooling expected.

When an on-board heater is fitted separate supply temperature sensors are available before and after the heater.

Two compensation curves are provided, one for heating and one for cooling. 

This means one could add separate compensation factors for heating and cooling.

The heating compensation factor can therefore include compensation for stratification.

In short the system checks how far supply air is above or below 22 deg C. If a compensation value of 1 deg C is added, the system will add for the cooling curve and subtract for the heating curve 1 deg C for every 10 deg difference between supply   and 22 deg C.

Example: If supply is 42 deg C and compensation is set at 2 deg C, then 4 deg C will be deducted from the on-board sensor value.

Example: If supply is 12 deg C and compensation is set at 1 deg C, Then 1 deg C will be added to the on-board sensor value.