Regarding tuning of PID controller

I am currently making a PID based temperature controller.
This is regarding tuning of a PID controller.
When I say Tuning of PID controller ,does it imply that I am tuning it for one particular value of temperature?As in are the constants Kp,Ki and Kd valid only if I am working at a particular temperature or is it that I am finding Kp,Ki,Kd for my given system at a particular temperature but they will be valid for entire range of temperature?

You tune a PID controller for a particular control loop (with current hardware installed making up the loop). Once the loop is tuned you may change set-points as desired. You will need to re-tune the loop if you make changes to your mechanical hardware, i.e. when changing a final control element.

So suppose if I want to create a temperature ramp,then too I need to tune it only once?

By mechanical hardware you mean the setup right?heating coil,temp sensor and chamber?

Tuning a PID control loop is usually not a once-off matter. It takes time, depending on the method used for tuning.

Hardware is all equipment forming part of the loop. Temperature sensor, (PT100, thermocouple etc.) Electronic medium converting the temperature signal into the format used by the PID controller (for example a PLC will have an 4..20mA analogue input card, converting the mA signal to useful Real signal understood by the PLC internal software. The PID controller will measure the PV (Process Variable) and derive a MV (Manipulated Variable) that will be transmitted back to the final control element (e.g. a control valve, a Variable speed drive, a variable heating element etc.)

This loop (PV in, PID calculate, MV out) is repeated many times per second (depending on how the PLC cycle time was configured).

Once a PID loop is tuned, (usually done by making many set-point changes during the tuning procedure) the controller should be capable of handling the full range that the measured element can cover. We don’t tune a temperature controller to operate between 100 and 110°C and then re-tune for 110-120°C. Once tuned, you can change the set-point from 90°C to 120°C and the controller should simply react to get the PV to the required SP.

PID controllers are there to keep the PV at one required SP. As for a ramp control, I don’t think PID controllers would be the correct controller for that. In fact, I don’t know of a dedicated ramp controller per say. When changing SP from one value to another, the “ramp” rate between the two values is usually achieved by the PID controller in its attempt to reach the new SP.