Open the brackets using the correct verb form.
In fact, the Spiral concept ( to propose) ответ
as long ago as the 19th century.
Only the lack of suitable materials and manufacturing techniques (to delay) ответ
its development into a fully-fledged product until the 1930s.
Since that time, acceptance of the Spiral (to see) ответ
steady growth and this exchanger type (to use) ответ
today in many industries including chemical, steel and pulp and paper.
The first Spiral Heat Exchanger (to be) ответ
extremely simple in concept.
It (to consist) ответ
of two metal strips, bent into a nearly circular shape to form two concentric channels through which media would flow in opposite directions.
Channel spacing (to achieve) ответ
by a steel bar along the length.
1.was proposing
2.delayed
3.have seen
4.is used
5.was
6.consisted
7.was achieved