When the first three seasons of ``Seinfeld’’ come out on DVD on Tuesday, they are expected to become the biggest-selling DVD sets of a TV show ever.

And the big sales of ``Seinfeld’’ are just a reflection of what has become a phenomenon: the unexpected success of television series on DVD.

Remembering the very limited appeal of TV show compilations in the VHS home video format, no one thought consumers would shell out upward of $100 for a single-season set of ``All in the Family’’ or ``I Love Lucy’’ or anything else. But such sales have gone from $880 million in 2002 to a projected $2.3 billion this year, making TV DVDs the fastest-growing segment of the $15 billion-a-year DVD business.

By 2008, sales could hit $3.9 billion, according to Merrill Lynch media analyst Jessica Reif Cohen.

``For people who love TV — and there are a lot of them — the DVD set is the greatest thing to come along since the invention of the VCR,’’ said Robert Thompson, a professor of media and popular culture at Syracuse University.

``People are buying and buying and buying,’’ said Judith McCourt, the research director of Video Store Magazine. ``Consumers love it, studios have it, and it’s a moneymaker.’’

The reasons most often cited for this surge in sales:

• Viewers get passionate about TV shows.

The very first shows to take off in the DVD format — ``Buffy the Vampire Slayer’’ and ``The X-Files’’ — were those with followings whose devotion bordered on obsession. For these people, the DVD is technology catching up with passion, since the format allows them to skip easily to favorite episodes or scenes and often provides detailed ``extras’’ on the making of the series. This is one reason why DVDs are catching on where VHS tapes didn’t.

Even if a network or cable show has a relatively small audience and lasts only a handful of episodes, a passionate fan base can translate into big DVD sales. ``When you look at the TV ratings of failed shows, they are still in 3 or 4 million homes,’’ said Mike Saksa, vice president of marketing for Warner Home Video.

``That may not be enough to get a show renewed, but we only have to reach one out of eight of those 4 million viewers to have a highly successful DVD.’’

• People watch some TV shows for the first time on DVD.

One of the most successful distributors of TV DVDs is HBO, the premium cable channel that has struck gold with sets of ``Sex and the City’’ and ``The Sopranos.’’

Even though she has never subscribed to HBO, Rachel Wrightman, a 31-year-old real estate agent from Los Gatos, said she has seen every episode of the first five seasons of ``Sex and the City’’ on DVD, and is eagerly awaiting season six.

``I just didn’t feel right about spending the money to buy HBO but a friend really sold me on the series,’’ said Wrightman. On DVD, ``I can watch whenever I want and watch the episodes I really like more than once.’’

Thompson noted that ``there a lot of shows out there right now for which DVD is the perfect medium. I don’t watch `24’ when it’s on TV anymore because it’s too hard to remember to keep up with it or to tape it. I watch it all at once when it comes out on DVD. In terms of artistic impact, there’s something to be said for watching all of one season in a weekend binge.’’

Still, networks and TV show producers are finding that DVD sales don’t hurt series still on the air. In fact, they can help — particularly with complex serialized dramas. After getting only so-so ratings its first season, Fox rushed a ``24’’ DVD package into the stores and saw the show’s viewership jump by almost 25 percent the following season.

• The DVD format makes having a home library of old TV shows practical.

``Home video was never effective for TV shows’’ because of the number of tapes needed, even for just one season, said Thompson. ``To go out and buy a whole season of a show meant filling up your entire shopping cart or a whole shelf in your house.’’

Now, said 37-year-old Joel Lynn, a self-described ``TV junkie’’ from San Jose, ``I can keep full seasons of all the series I want without being overwhelmed, just like I have a bunch of CDs.’’

The bestselling TV DVD sets still sell a fraction of the copies that a blockbuster movie will. The top film on the bestseller list, ``Finding Nemo,’’ has sold 22 million units; the No. 1 TV set — the first season of ``The Simpsons’’ — has sold 2.35 million.

And some DVD sets have been surprising busts: The first season of ``The Mary Tyler Moore Show’’ did little business and, though not a money loser, the first season of the highly rated ``CSI’’ didn’t sell anywhere near what was expected.

But overall, the volume of sales has been escalating. Since its release in late February, the first season of ``Chappelle’s Show,’’ a Comedy Central series featuring comedian Dave Chappelle, already has sold 2 million copies. ``Sex and the City: Fourth Season’’ was the 10th bestselling DVD on Amazon last year.

In addition, the profit margin on TV DVD sets tends to be considerably more than on DVDs of big feature films. Studios make anywhere from $8 to $30 on a TV DVD set. The break-even point for Paramount Home Video on ``Chappelle’s Show’’ was around 13,000 copies. Estimates are that the Chappelle DVD will make $54 million in profit.

Which explains why even short-lived, relatively obscure shows are coming out on DVD — where they often are proving to be hits. As just one example: ``Battlestar Galactica’’ aired just 24 episodes on ABC from 1978 to 1980 but its DVD set has sold more than 150,000 copies — at more than $100 a set.

As a result, some very good recent shows that did not find an audience are getting a second life, much to the delight of their creators. Joss Whedon, whose ``Firefly’’ series lasted less than half a season on Fox two years ago but has seen big sales on DVD, said, ``To know that anybody can go to a store and pick it up — and see what I think is some of the most compelling stuff I’ve ever done — is wonderful. It’s not lost; it’s not stillborn; it exists.’’

``Firefly’’ may be one of the better examples of the ripple effect DVD sales are having on Hollywood: Largely as a result of the DVD sales, Whedon has been given a $100 million budget to make ``Serenity,’’ a theatrical film version of the series. Another series — the animated ``Family Guy,’’ canceled by Fox in 2002 — has done so well on DVD that it will be reborn as a series next year.

And in an example of the DVD and broadcast worlds feeding off each other, NBC will air a Thanksgiving night special called ``The `Seinfeld’ Story’’ (10 p.m. Thursday, Chs. 8, 11) which not only is studded with bits from the DVD extras but is really one long commercial for the set.

Not, as they used to say on the show, that there’s anything wrong with that.