I am trying to add where in collection but it return 

> Varien_Db_Select Object

instead of return collection object.

    $collection = Mage::getModel('orderoutofuk/orderitem')->getCollection()->getSelect()->where('main_table.send_payment_request_date >= DATE(NOW()) - INTERVAL 7 DAY ');

and I want specific column value in array instead of looping through each object.