There are commercial liquid nitrogen guns used by dermatologists to freeze warts and skin cancer off. It is believed they have a pressure release valve that keeps the pressure at around 70psi. The problem is its hard to make a thermos thats also pressure rated because you have a pressure on the inside of the flask, then a vacuum directly on the other side. They are fragile and expensive. He tried to put a nichrome heating element inside the thermos to increase pressure, but it ended up not working that great. shaking the thermos made way more pressure!