Esbit stoves dont work well at all. I had one in the army for years, and it never did any better than luke-warm. Someone else recently asked a thread about this same thing.
There are far better methods than an esbit stove. The trioxane tabs are great for starting a fire, THEN boiling water. As a stove, it doesnt work that well. Even with a windscreen/heat reflector. If youre using it simply to warm meals, its fantastic though. Just dont count on it to purify water.