Question in DAC (k8055)

Hello ,
i am student in faculty of engineering ,cairo university and i have a question in DAC (K8055)that use in my project:-
My process in the following summery:-
i had made tempreture control using this DAC through reading the tempreture via tempreture sensor (k6001)that produce PWM signal propotional to measured tempreture and then send it to SCADA system that has setpiont and PID inside it so it will produce control output to pwm pin in DAC that will connected to relay to heater with external supply source(actuator) to supply this heater with current =6.8 AMP not depend on current drawn from DAC
the problem:-
What is the frequency of PWM of DAC output to relay to heater?
i hope the frequecy equal power freq. in egypt =50HZ so now i can use our available relays with 5o HZ not use transistor or mosfet with high freq.

thanx in advance ,i hope reply becuase it is very very important for me please

The frequency is fixed 23kHz and the duty cycle is proportional to the corresponding analog output value.