Over the next 25 years, he transformed GEA into a global system supplier and introduced machinery and plants for the food processing industry, Energy industry, air-conditioning and refrigeration industry. After making over 70 acquisitions, GEA had over 200 subsidiaries in 60 countries. In 1989 he took his company public at the Frankfurt stock exchange, but kept the majority of the voting stock in the family. By 1999, Happel had increased GEA's revenues to over €2.56 billion, as well as its net earnings proportionally.
Happel maintains his family office Luserve AG in Lucerne, Switzerland. He served as a member of the supervisory board at Commerzbank from May 1993 until April 2013.
In 1999 he sold his controlling stake to Germany’s MG Technologies, became the major shareholder of MG Technologies and, once MG Technologies was merged into GEA Group AG, sold his remaining stake to institutional Investors in 2006. GEA Group AG is the largest listed German machinery manufacturer and is broadly Diversified. In 2016, it accounted for revenues of approximately EUR 4,5 bn and employed approximately 17,000 people.
As of March 2017, Forbes estimated his net worth at US$2.7 billion.