Direct Acting Thermostat Definition

On the other hand the reverse acting pneumatic thermostat or ra is responsible for decreasing the pressure in the branch line as temperature is increased.

Direct acting thermostat definition.

The thermostat that controls a typical house furnace is reverse acting since it switches the furnace off or de energises as the set point is reached. Direct acting and reverse acting my comments the terms direct and reverse are frequently used when discussing control valves positioners and controllers while writing the control narratives for the complex loops though the definitions of direct and reverse seem pretty straightforward. How direct acting and reverse acting control valve loops work custom search a typical control valve loop consists of four basic elements for consideration. This week s podcast covered the difference between direct acting and reverse acting thermostats and controllers.

A the process under control mainly flow or pressure applications b the process controller that initiates control action to the control valve. As i understand it if the controller output increases when the measurement value rises it is a direct acting controller and if controller output decreases when the measurement rises it is a reverse acting controller. Direct acting types of thermostats increase the pressure in the tube in proportion to the rise in temperature. A reverse acting ra pneumatic thermostat decreases branch line pressure as temperature increases.

The opposite to this is a direct acting thermostat which energises equipment as the set point is reached. In a reverse acting pneumatic thermostat the pressure lowers when the temperature increases while rises in pressure occur as it gets cooler. In addition at minimum output of the controller the desired fail safe action must be achieved. A direct acting da pneumatic thermostat increases branch line pressure as temperature increases.

Mcgraw hill dictionary of architecture and construction. Direct acting thermostat an instrument which activates a control circuit when a predetermined temperature is reached. For example if we have two controllers on a distillation column reflux.