Best practice no. 24

When the heat demand decreases due to the needs of the process, the control valve acts to prevent overheating of the fluid to be heated, reducing its opening, which increases the pressure loss produced by the valve. This reduces the steam pressure inside the heat exchanger and therefore its temperature, thus reducing the heat flow to the product. If this decrease in vapor pressure becomes large enough, it may occur that the vapor pressure inside the heat exchanger is lower than the pressure in the condensate line. At that moment the differential pressure becomes negative, and the trap will stop draining condensate: blockage begins to occur, and condensate will begin to accumulate inside the heat exchanger. It is common practice for designers to oversize the surface of heat exchangers, to consider the foreseeable fouling of these, as well as possible additional needs due to future changes in the process. This oversizing is usually at least 20%, with oversizing of the order of 40% or even higher not being strange. An oversized heat exchanger (as most heat exchangers are) tends to operate with relatively small control valve openings, and therefore with relatively low steam pressures within the heat exchanger. It is even possible that under nominal load conditions, blocking may occur if the exchanger oversizing is very large. Blockage of a heat exchanger has various negative consequences on the equipment and the efficiency of the process. We will see some causes before seeing the solutions to blockage in heat exchangers: 1) Loss of productivity: if the heat exchanger becomes blocked, heat transfer will stop occurring, with the consequent impact on the productivity of the heat exchanger and possibly the entire process. Normally, when this happens, the bypass valve of the steam trap is opened manually and most of the time it is forgotten to put the steam trap back in the line, with the consequent increase in steam consumption, much of which will be wasted. 2) Uneven heat transfer and quality loss: With blocking, the heat transfer may become uneven, so the product may have quality problems because the heating will not be equal at all points. Effect on blocking oversizing of heat exchangers What happens if my heat exchanger blocks?