3. Use flexing elements: a heating element made of thin gauge monel metal built like a concertina may be used. The advantage of such an element is that when pressure, and hence temperature, vary slightly the element flexes considerably thereby cracking off scale effectively and permitting longer running of the evaporator between shutdowns. periods be taken to ensure it However, when such a element is used care must is not subjected to high pressures otherwise failure can occur. A safeguard consisting of an air-operated trip valve in the steam line, controlle by a solenoid pilot valve actuated by a differential pressure differential pressure gauge, which registers across the element (maxim o.9 bar pressure difference. may be fitted.