Sift together the flour, baking powder, baking soda and salt. With the electric beater on low, fluff the shortening in a small bowl, until soft and creamy. Add the sugar gradually. Beat in the eggs in a slow stream.  With a fork, beat in 1/3 of the flour mixture, 1/2 the bananas another 1/3 of the flour mixture, the rest of the bananas then the last of the flour mixture. Fold in the walnuts. Turn into a greased and floured baking unit or a 2 1/2 quart mold and cover. Place on a rack in the slow cooker. Cover the cooker, but prop the lid open with a toothpick or a twist of foil to let the excess steam escape. Cook on high for 4 to 6 hours. Cool on a rack for 10 minutes. Serve Warm.