cascade system

[next life]

hello
aim now try to build cascade cycle to reach -70 c
the high stage work on R404a ,using tecumseh compressor 1/2 HP
the low stage work on R23 ,using tecumseh compressor 1/4 HP
i used capillary tube for both stages
but my problem is in the heat exchanger , i don't know how to select the heat exchanger ,
the calculation show that the heat exchanger capacity is 900 w and to reach -70 , i soppuse to use tube in tube heat exchanger ( 6 m long , diameter for inner tube is 3 mm and the diameter of the outlet tube 6 mm ) is that OK ? or there is a problem in ,y calculations ?

[mbc]

could you draw you system.
and I think it is more safe you use plate heat exchange .