What are the returns on a PPF account?

What I gathered is if you invest Rs.70,000/- p.a, i.e.(Rs.1050000 for 15 years) you get around Rs. 20,00,000/- after 15 years. Is it correct? I thought the amount would be around Rs. 28,00,000/- Please guide.
Will the same interest rate at which I opened the account prevail, throughout the 15 year tenure or would the rate of interest vary yearly?