In a large saucepan, combine the can of chicken broth, the diced potatoes, onion, celery, cayanne powder, garlic powder, the chicken boullion powder and the water. Bring to a boil, reduce heat and simmer until the veggies are tender. Combine 1/4 cup of the evaporated skim milk with the 3 tablespoons cornstarch and set aside. Add the remainder of the evaporated milk, the clams with their liquid, and the cornstarch mixture to the pan with the veggies. Cook and stir until thickened. Season to taste with salt and pepper.