Limit a WordPress feed to one post type.

feed_filter.php

<?php
function my_feed_filter($query) {
    if ( $query->is_feed && 'cpt_shop' === $query->get('post_type') ) {
		$query->set('order', 'asc');
		$query->set('meta_key', '_alt_title');
		$query->set('orderby', 'meta_value');
    };
    return $query;
};
add_filter('pre_get_posts','my_feed_filter');