What is the optimal forced air flow on the kiln shell? How does this impact on the refractory life? What is the interval in the shell temperature when I should turn on and turn off a fan?

I would say the optimum forced air flow on the kiln shell is zero. The higher the air flow then the higher is the loss of energy through the shell. This means greater heat requirements in the kiln and more penetration of salts into the refractory and therefore a negative impact on refractory life. However, as the refractory lining wears switching a fan onto the shell can be useful to try to build coating. I would suggest switching the fans on at above 350 degrees C.

We carry out refractory work with screw jacks, maximum 10m lining at a time (ie 5m maximum simultaneously at two different locations in same kiln). As ours is a small plant of 1000tpd, with single kiln of 64m length, deploying a brick lining machine and dismantling machines are not viable in terms of cost for us. What therefore is the maximum length of refractory work can be done at a time, in how many parts, simultaneously, from the various aspects such as safety, life of refractory etc.

I would recommend using the glueing method rather than screw jacks and then you can replace as long a section as you like without the concern of installing and removing the jacks. My understanding is that the glueing method will have no problems with a kiln of 3.95m diameter. However, you would be better to check this with suppliers of the refractory glue when placing your order.