The Postal Service ushered itself into the computer age today, unveiling a 30-foot-long machine that reads addresses and zip codes and promises to sort letters so fast that future postal rate increases might be delayed.

U.S. Postmaster General William F. Bolger took reporters on a tour of the post office terminal annex here, where the first of the system's optical character sorters (OCRs) and bar-code sorters (BCRs) have been in operation for two months, firing out letters at the rate of 28,000 an hour.

The machines, made by Pitney Bowes Corp., Burroughs Corp. and Bell and Howell Corp., and with some Japanese and Italian technology, are to be installed eventually in 211 postal facilities at a cost of $690 million. Bolger said they would save $600 million a year in labor costs alone.

The last postal rate increase, to 20 cents for a first-class stamp, occurred in 1981. Bolger said he hoped the new system would help stretch what has been a 2 1/2-to-3-year cycle of rate increases.

The new sorting machines, attached to computers and long series of trays for letters bound to different cities, electronically read addresses or zip codes, ink a supermarket-style bar code on each envelope and hurl the mail down a long horizontal slot.

Supervisor Georgia Booker said she worked seven years reading and punching zip codes into the mechanized keyboard sorting machines now used. But she noted that keyboard operators could sort only about 1,850 letters an hour that way, while the efficiency rate of two or three workers handling each of the new machines is five times greater.

Bolger said the new system is designed principally for business mailers, who send 83 percent of the nation's letters and receive, often in the form of pre-addressed envelopes, another 11 percent.

Americans' personal letters, although only 6 percent of the total, may be sorted by computer if senders have properly typed the city, state and zip code on the bottom line of the address and left enough space in the lower right corner for the machine to ink the bar code. Bolger said he had seen one advanced sorting machine, not yet in use, that could read 62 percent of handwritten addresses with zip codes.

Bolger said the Postal Service is not promising faster delivery than its current standard of three days from New York to Los Angeles and one day on some routes of fewer than 350 miles.

"But for all mailers the tangible payoffs from automation will come in the form of more consistent service, fewer sorting errors and sharply reduced operating costs for the Postal Service," he said. This, he said, "will help us hold down and spread out future rate increases."

"No postal employe will lose his or her jobs due to the latest automations," he said.

Moe Biller, president of the 320,000-member American Postal Workers Union, said he welcomes Bolger's guarantee against layoffs but said the new machines had not been tested sufficiently to determine if they pose problems for workers or the postal system.

Although the new system will begin to use the new nine-digit zip code in October, Bolger said that businesses and individuals could ignore the new code if they chose. Mass first-class mailings that use the new code may get a discount in the future, however.