Problems with steam-heated reboilers

The steam should of course be superheated steam. During heating, the ideal condition in the reboiler is half steam and half water, that is, phase-change heating; it’s best to install a drain valve as well

The reboiler is heated with steam that is saturated; a baffle is installed at the inlet to prevent water hammer. The low-pressure steam in refineries is both saturated and superheated. When heating the reboiler with steam, the reboiler contains a small amount of condensed water at the bottom and steam at the top, and it is heated by the heat of phase change.

Using saturated steam is good; there will be no water hammer. The heat transfer of steam in the reboiler can be simply divided into three stages. In the first stage, the superheated gas is cooled to saturated gas; since the sensible heat of the gas is very low, the amount of heat released in this stage is small. Moreover, because the convective heat transfer coefficient of the gas is low, a large area is required for this stage ； In the second stage, saturated gas condenses; due to the large latent heat of vaporization, significant heat is released during this stage. Moreover, because the heat transfer coefficient for condensation is high, less area is required for this stage. In the third stage, the saturated liquid cools down; since the sensible heat of the liquid is not very large, the amount of heat released in this stage is also limited. The volume of liquid that condenses is small, the flow velocity is low, and the convective heat transfer coefficient is not high either, so a larger area is needed for this stage. From the perspective of reducing reboiler investment, it is of course necessary to reduce the proportion of the first and third stages; therefore, saturated steam should be used for heating, and the liquid level should not be too high.

1. When heating the reboiler with steam, should it be using saturated steam or superheated steam? Generally, superheated steam is used. 2. If a saturated solution is used, will it cause water hammer? It will not cause water hammer. 3. Is the low-pressure steam in the refinery saturated or superheated? It must be overheated; otherwise, a large amount of water will condense during transportation. 4. When heating the reboiler with steam, is the reboiler filled with condensate at the bottom and steam at the top, and is heating achieved through phase change heat? It must be phase change. Because when steam at 100 DegC turns into water at 100 DegC, a large amount of heat is released, and this is enthalpy.

Saturated steam has little latent heat; relying on its sensible heat is generally not sufficient to meet the heat exchange requirements. Superheated steam should be used, and it is important to install traps, as superheated steam provides the most heat when it is cooled to a temperature of saturated temperature plus 5°C. Therefore, the use of traps is particularly crucial. Condensate or a mixture of liquid and vapor usually does not cause water hammer, but issues such as air blockages can occur. Poor drainage of condensate can be resolved by installing traps. This post was last edited by bluetop on 2008-1-22 at 23:41

When designing, we generally use saturated steam in reboilers, as the difference between sensible heat and latent heat is quite large. With the same heat load and the same heat exchanger size, I tried using HTRI by changing only the steam temperature, and the result was that it was not as effective as using saturated steam. Please offer some guidance, let’s learn from each other

The heat transfer coefficient during condensation with phase change differs significantly from that without phase change; moreover, steam at higher temperatures has a lower latent heat, so less heat is transferred during the phase-change stage characterized by a high heat transfer coefficient； Saturated steam has a high latent heat, so more heat is released during the phase change stage with a high heat transfer coefficient, resulting in good performance. This is why you arrived at this conclusion. Heat exchangers that use steam for heating adjust the area available for phase-change heat transfer by changing the liquid level in the submerged tubes; by reducing this area for efficient heat transfer, the overall heat transfer efficiency is decreased, thereby enabling control of the heat exchange load. A steam trap is generally used at the outlet of the heat exchanger to reduce energy consumption and prevent water hammer. For large quantities, a water tank can be connected.

Using saturated steam is good; there will be no water hammer.

Theoretically, it is better to use saturated steam. However, in actual production, it is better for the steam supplied to the equipment to be slightly superheated. This prevents liquid slugging from occurring in the device due to the condensation of saturated steam in the pipeline delivery system.

Use superheated steam if available; it has a high calorific value and enables rapid heating

Using superheated steam will cause water hammer, right? What about saturated steam? Is it related to the temperature and pressure of the reboiler?

A reboiler heated by steam will not experience water hammer, unless there is a problem with the design of the reboiler. Whether to use saturated or superheated steam depends on your actual production conditions and process requirements. Superheated steam has very low sensible heat; the heating process relies mainly on the latent heat released during phase change. Forcing its use in the absence of superheated steam leads to energy waste, and if it is possible to make comprehensive use of it, then superheated steam should be considered

The selection of the control scheme is quite important; for different types of reboilers, the control temperature difference should be around 50 degrees.

After the steam condenses, by controlling the amount of water removed, it is ensured that a portion of the tubes inside the heat exchanger remains submerged in the condensed water. This changes the area of the condensation zone; since most of the heat is transferred through condensation, changing the area of the condensation zone is equivalent to altering the effective heat exchange area for condensation, thereby enabling control over the heat exchange efficiency. A heat exchanger designed in this way can maintain the pressure inside it at a normal level, but due to the need to take into account the area of the flooded tubes, the design margin of the heat exchanger is quite large, which increases its cost. Another control scheme is to control the temperature by regulating the inlet flow rate of steam. In this case, the pressure inside the heat exchanger can experience significant fluctuations in the steam side pressure due to the operation of the control valve and possible rapid condensation, resulting in alternating stresses that are slightly detrimental to the sealing of the heat exchanger.

In the reboiler I designed this time, the material to be heated is heated using two different pressures of steam: 10 Bar and 25 Bar. Both are saturated steam. In my opinion, it depends on the properties of the material to decide whether to use saturated or superheated steam. Steam enters from the top, while condensate water enters from the bottom; moreover, determining whether the material flows through the tube side or the shell side is also a key factor

The original post was published by tianwentang on 2008-1-23 at 12:07. Theoretically, it is better to use saturated steam. However, in actual production, it is better for the steam supplied to the equipment to be slightly superheated. This prevents liquid slugging from occurring in the device due to the condensation of saturated steam in the pipeline delivery system. [/quot I agree with the above opinions; that’s exactly how we proceed during the production process. I have another question: Is it true that using superheated steam has an impact on the lifespan of equipment?

What is water hammer? Is it the same as water hammer?

The heat transfer control of the reboiler can be achieved by controlling the steam flow rate, or by controlling the steam pressure inside the reboiler. The steam flow rate is high, resulting in a high outlet temperature of the material being heated. If I want to increase the outlet temperature of the material being heated, should I increase or decrease the pressure? Question: In a reboiler equipped with a condensate tank, the level control of the condensate tank is crucial for the stable operation of the reboiler. Is this related to the reboiler level mentioned earlier? Is there any correlation between changes in the level of the condensate tank and the outlet temperature of the heating medium used in the reboiler?

Are you referring to the pressure on the steam side, or on the backup heating side? Changing the liquid level changes the heat exchange area of the evaporator, which in turn changes the amount of heat exchanged.

We used saturated steam, with flow from high to low, and a hydroseparator was installed afterward; no water hammer phenomenon occurred