hvac

None.

what purpose we are using modulating valve and balancing valve

Usually, maintaining temperature.

What "we" are doing with them cannot be seen from here.

any one provide the images for modulating and balancing valve.

http://uk.ask.com/pictures?q=modulating+and+balancing+valve&search=&qsrc=0&o=0&l=dir&siteid=

Hi maglak.

There is a profound difference in the modus operandii of the two valves.

a) The balancing valve is used to set the relative flows in different parts of an hydraulic circuit. Typically they are set once,during the commissioning stage of the project, whilst readings are taken from pressure impulse connections incorporated into the valve body( the delta P is monitored by a hand held instrument which converts this into a flow rate).Havig achieved the design flow rate the valve is then locked in that position. As there may well be several such valves on a system it is important to check all flows if adjustments are ever made to the network or even a pump is changed.

b) The modulating valve functions as per its name implies and through positional control is used to vary the quantity of water passing to a particular load item eg heater /cooler battery, radiator system etc. This valve therefore either has a linear stem motion or a rotary shoe arrangement either of which is connected physically to an appropriate style of actuator which in turn responds to a control circuit with or without feedback (pulsed , 4-20mA or 0-10V signal).This postional control reflects the deviation from setpoint of the measured variable ( temperature, pressure, humidity etc).

The valves are different in construction and have different hydraulic characteristics; therefore you would never try to substitute one for the other!!

If you want more info look up the CIBSE web site.

Good luck,

Massey