Best practice no. 24

February 2024

Number 24

BEST PRACTICE

Stall Efect in heat exchangers

In heat exchangers in which a fluid is heated by steam, the phenomenon called “blocking” often occurs, which in the long run can lead to various problems, including significant mechanical damage to the heat exchanger. heat. Today we will talk about the solutions to blockage in heat exchangers. Blocking occurs when heat demand falls below a certain critical level. For example, because, due to process needs, the flow rate of the fluid to be heated is reduced; if this reduction is large enough, blocking may occur. The blockage consists of the accumulation of condensate inside the heat exchanger because the trap is not able to drain it. The figure shows an illustrative image of the phenomenon, indicating the accumulated condensate (blue) and the vapor (violet) which, as can be seen, does not cover the entire bundle of tubes, which reduces or even makes heat exchange impossible. In heat exchangers that use steam as a heat source, the steam flow is usually regulated by a control valve equipped with a pneumatic or electric actuator. As the heat demand varies, the control valve opens or closes to maintain the temperature of the fluid being heated within established limits. These variations in valve opening translate into variations in the pressure and temperature of the steam within the heat exchanger. Let's look at an example: if the steam supply pressure is 6 kg/cm² (corresponding to 165°C), the fully open control valve produces a pressure loss of 2 kg/cm² and the pipe that connects the latter with the exchanger has a pressure drop of 0.5 kg/cm², the effective steam pressure inside the heat exchanger will be 6 – 2 – 0.5 = 3.5 kg/cm² and the steam temperature will be 148°C, not of 165°C as we might mistakenly imagine. A second aspect to consider is the effect that variations in heat demand have on the opening of the control valve and consequently on the pressure loss generated in the steam and on its effective pressure inside the exchanger of heat. Why are heat exchangers blocked?

P2

P1

P3

P5

P4

P6

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?

3) Non-use of the heat transfer area: blocking the heat exchanger decreases the effective heat transfer area. The heat exchanger is not fully utilized and at some point, it has been observed that to make up for the shortfall an additional heat exchanger is added in order to overcome the problem. 4) Water hammer: When the equipment load drops (due to factors such as a reduction in the amount of product to be heated or an increase in product temperature), the pressure differential between the inlet and outlet pressures of the pressure trap steam disappears, blocking of the heat exchanger begins and condensate begins to accumulate in the casing. With a reduced (or no) steam supply, the accumulated condensate cools, and when the control valve opens again and steam is supplied, it, in contact with the cold condensate, condenses instantly and a water hammer is produced. In most cases the water hammer generated by this mechanism is not very violent, resulting in small but perceptible impacts for a short period, unlike the violent impacts that occur in steam distribution lines when it occurs in them a water hammer. However, if these small-scale water hammers are allowed to continue for long periods of time, they can weaken equipment to the point that it suddenly breaks. This breakage often occurs under conditions of high pressure, heavy load, and full operation. Therefore, rapid condensate discharge is essential from a preventive maintenance point of view. 5) Heat exchanger leaks: Due to water hammer and corrosion caused by condensate buildup, there is a possibility of heat exchanger leaks and failures. Uneven Temperature Sometimes we believe that only heat exchangers have this blocking effect, also kettles can lead to serious problems of non-uniform temperatures in the product. When stall occurs, condensate accumulates. The temperature of the lowest part of the condensate accumulated inside the jacket drops, experiencing uneven temperatures on the heat transfer surface, which can have a negative effect on the quality of the product. Replacing the steam trap does not prevent a stall effect since it will not resolve the negative differential pressure that occurs across the trap.

Steam line

Condensate return

Preventing lock-in effect usually includes two methods:

• Increasing the pressure at the trap entrance (primary), or • Reducing the pressure at the trap outlet (secondary).

Unequal Temperatures Occur at the Heating Source (Heat Transfer Surface)

How to Prevent the Stall Effect Reducing the outlet pressure requires the use of a vacuum pump. When a vacuum pump is used, the pressure in the trap outlet pipe (condensate return line) is reduced below atmospheric pressure, this maintains the differential pressure that is required for the trap to operate. When using a vacuum pump, the outlet pressure of the steam trap is lower than the inlet pressure and allows condensate to be discharged through the trap. Depending on the capacity of the vacuum pump, several traps could be handled. • Steam trap that discharges to a flashing tank with a mechanical pump. • Steam trap that discharges to a flash tank with a motorized pump. When we use a pump or the trap/mechanical pump combination, pressurized air or steam is applied to the accumulated condensate to increase the inlet (primary) pressure so that it is greater than the outlet (secondary) pressure of the trap. This will force condensate to discharge before it accumulates inside the equipment. By installing a mechanical pump equipped with an internal trap, and using steam pressure from another line, it is possible to discharge the condensate immediately without any accumulation inside the equipment (in the heat transfer zone). This type of pump/trap also eliminates any fear of cavitation. Increased inlet pressure can be achieved by changing the system configuration to include any of the following: • Mechanical Trap/Pump.

Vacuum pump

Mechanical trap/pump

By installing a mechanical pump, with the use of a steam trap that discharges to a flashing tank, it is possible to discharge condensate from the trap as the trap's secondary pressure (back pressure) is reduced. The pump is used when the condensate is returned to an elevated location. Likewise, this equipment eliminates any fear of cavitation.

La contrapresión en pequeña

Mechanical pump

By installing a mechanical pump, with the use of a steam trap that discharges to a flashing tank, it is possible to discharge condensate from the trap as the trap's secondary pressure (back pressure) is reduced. The pump is used when the condensate is returned to an elevated location. Likewise, this equipment eliminates any fear of cavitation.

La contrapresión en pequeña

Motorized pump