After a week in Washington the 900 people attending the National Association of Home Builders' spring meeting went home Wednesday.

Most of the 2,000 persums from the American Helicopter Society have checked out of the Sheraton Park, too, just in time for 550 YMCA people to check in.

The 3,500 doctors and spouses of the American Urological Association are coming Sunday, but 175 clinical unrologisgs are already here for their specialized sessions.

The B'Nai B'rith women will be in Saturday, the newsiemer publishers are arriving Sunday and the American College of Sports Medicine opens its session Wednesday. Next Saturday there will be 2,000 people here for the National Catholic Forensic League, and another 600 for the Three Diamond Society.

May is a good month for conventions in Washington and Washington is a good city for conventions.

An estimated 715,009 delegates attended 824 major conventions in Washington last year, says Austin Kemy, executive vice president of the Washington Area Convention and Visitor Association.

The conventioners stayed an average of 4.4 days, spent about $300 apiece and poured $215 million into the city's economy.

That doesn't count the money companies spent on hospitality suites, or the cost of setting up exhibits or any amount of convention registration fees that found its way into Washington spending.

Those expenditures swell the dollars per conventioner toward the $400 mark, estimates Kenny, who knows more about Washington conventions than anybody else in town.

Conventions are a basic industry in Washington, he contends, bringing out-of-town money into every motor lodge, taxi and tourist trap.

Convention statistics are prone to distortions born of boosterism, Kenny admits. He says he has a stock of questionnaires filled out by convention delegates that convinced the General Accounting Office that his convention spending figures are solid, and probably conservative.

As for total convention business, there are at least half a dozen cities that compete with Washington. Kenny figures any convention that doesn't draw 100 people doesn't count, although other cities draw the line at 50.

In the category of national conventions, Washington ranks second or third, neck and neck with New York, he argues. Atlanta, San Francisco and other towns may have more total convention delegates because they draw big regional crowds but for national meetings, Washington is ahead.

Everybody in the convention business concedes that Chicago is number one and its McCormick Place often is described as the best facility for a big convention or trade show.

Despite Lake Michigan "breezes" that turn McCornick's entry into an Arctic wind tumel for months at a time and tear gas memories of the 1968 Democratic convention, Chicago remains preeminent because of its location and facilites.

By air, Chicago is more accessible than any other city in the country. It has more hotel rooms than any other, including the newest, 1,214 room Marriott that opened this week.

McCormick Place is also the most successful convention facility in the country, grossing $8.4 million in rentals and service charges in 1975. Unlike just about all its competitors, McCormick turns a profit from operations, $714,000 that year.

With 800,000 square feet of exhibit space and meeting rooms boxed inside its blcak beams, McCormick Place routinely hosts shows that are too big to go anywhere else.

Even if the proposed 300,000-square-foot convention center were built at Mount Vernon Square, Washington could not capture Chicago's big gate events, such as the annual houeswares shows that draw upwards of 20,000 persons.

Now, however, the District lacks the facilities to compete for many events and regularly loses business because of it, Kenny says.

The American Hospital Association, to name one trade group, has shopped Washington's facilities and decided it must meet elsewhere, despite its leaders' desire to convene in Washington.

There are 300 or 400 national organizations that put on major conventions or shows every year and more than 100 of them are too big to meet in Washington.

Typically these big shows drew 9,000 delegates, but require almost half that many exhibitors to staff up to 1,500 exhibition booths.

Among other groups that have outgrown D.C. are the National Micrographs Association meeting, which annually draws 12,000; the National Association of Printers and Lithographers, 18,000 attendees: the American Dental Association, 25,000; the American College of Cardiology, 14,000; the National Canners Association, 9,500, and the Automatic Car Wash Association, which draws only 6,000 members but requires mammoth exhibit space.

Washington's three biggest convention sites now are hotels: the Sheraton Park can handle about 4,500 people, with 64,000 square feet of exhibit space; the Washington Hilton's convention capacity is about 4,200, with 40,000 square feet of exhibit space, and the Shoreham can take 2,500 people and offers 30,000 square feet of exhibits.

The Sheraton currently is in the midst of a significant, $74 million expansion. When completed by the fall of 1979, its exhibit space will be doubled.

Only one convention, the National Association of Broadcasters, has tried tying together the three hotels for one convention. Transportation hassles and the fear of delegates and exhibitors that they won't be in the "best hotel" tends to discourage multi-siteconventions.

The big hotels aren't the only game in townfor conventions. There are about 15,000 hotel rooms in the District and 35,000 in the metropolitan area, most of them inside the Belt-way.

Local convention schedules show hundreds of smaller meetings in places like the three Marriotts just across the Potomac, the Ramada Inn in Old Towne Alexandria, the Arlington Hyatt, the International Inn, Loew's L'Enfant Plaza Hotel, and others facilities.

Just how heavily local hotels depend on conventions is shown by a survey of the 12 largest ones, taken by the convention association. The hotels identified about 15 percent of their business as tourists; 33 percent business travelers, mostly those doing business with the government, and the rest, just over 50 percent, conventions.

The hotel business and restaurants would be the prime beneficiaries if Washington builds the proposed Mount Vernon Square convention center. Because the food and lodging industries are labor intensive, the hotel and restaurant buiness generated by the center would create an estimated 4,000 new jobs, a figure disputed by center opponents as inflated.

Convention center supporters claim that the city would get between $91 and $115 million a year in new business and from $12 to $16 million in new tax revenue. Sales taxes on spending by conventioners are projected to produce $3.4 to $4.1 million, real estate taxes on private developments to serve delegates, another $2.5 to $3.4 million.

The 1976 feasibility study projects that the city would collect $3.1 million to $4 million in hotel occupancy taxes, a figure that does not include the 80 cents-a-room-a-night hotel tax since enacted.

The economic projections are based on the assumption that after a two or three-year warmup, the convention center would bring Washington 310,000 to 390,000 more convention delegates a year. That's in addition to a projected growth of 100,000 conventioners a year, whether or not the center is built.

Between 2,450 and 3,300 new hotel rooms would have to be built. At $35,000 per room - not counting land costs - the construction would cost nearly as much as the convention center itself, $85 million to $115 million.

Another 40 or 50 big restaurants would also be spawned, the feasibility study says.