Rails 3, apache & passenger, send_file sends zero byte files

I'm struggling with send_file with rails 3.0.9 running ruby 1.9, passenger 3.0.8 on apache on ubuntu lucid The xsendfile module is installed and loaded into apache

root~# a2enmod xsendfile
Module xsendfile already enabled

Its symlinked correctly in mods-enabled

lrwxrwxrwx 1 root root   32 Aug  8 11:20 xsendfile.load -> ../mods-available/xsendfile.load

config.action_dispatch.x_sendfile_header = "X-Sendfile" is set in my production.rb

using send_file results in zero byte files being sent to the browser

filepath = Rails.root.join('export',"#{filename}.csv")
if File.exists?(filepath)
  send_file filepath, :type => 'text/csv'
end