How did export surplus during british rule lead to the economic drain of Indian wealth?

During entire British colonial period,the value & volume of India's gross exports of primary goods (food grains, cotton, raw silk,wool,jute,sugar,indigo ,etc.) exceeded the gross imports of finished consumer goods like cotton silk & woolen clothes and capital goods like light machinery manufactured in British factories.This led to generation of huge export surplus. But this surplus came as a huge cost to Indian economy.Several essential commodities(food,clothes,kerosine) were scarcely available in domestic market.This also did not result in any flow of gold or silver into India(unlike pre -British rule) .Rather,this was used to make payments to the cost of British colonial establishment(Army,navy,bureaucracy,etc) in India,war expenses & expenses on imports of invisible items.This all led to irreversible drain of Indian wealth by the British raj.