For the benefit of others here is the solution which I worked out by experimenting.
Its very simple really.
I added the following 2 lines in the proftpd.config file:
TransferRate RETR 15.0
TransferRate STOR 55.0
This limits the download speed FROM my server to 15 Kbytes/sec
and the upload TO my server to 55 Kbytes/sec
Thats it. It works.