The cooling is usually attained by way of a refrigeration cycle, but from time to time evaporation or cost-free cooling is used. Air conditioning techniques can also be created based on desiccants (chemical substances which remove dampness with the air). Some AC methods reject or keep heat in subterranean pipes.[two] https://docs.google.com/spreadsheets/d/12u7rJLrjSWIMoTyNzJz1omJPJnoZX4A_U0YLwoyEYUk