One question per day – trial version; relatively simple. 2009-02-27

1 The latent heat of condensation of saturated steam is much greater than the sensible heat released when superheated steam cools down； 2 The heat transfer film coefficient for the condensation of saturated steam (with phase change) is much larger than that for the cooling of superheated steam (without phase change) ； 3 When the superheated steam temperature is too high, it has a certain impact on the service life of the equipment.

1 The latent heat of condensation of saturated steam is much greater than the sensible heat released when superheated steam cools down； 2 The heat transfer film coefficient for the condensation of saturated steam is much larger than that for the cooling of superheated steam ；

From the Qiushi Chemical Engineering Forum: Use saturated steam as a heating agent, making full use of the latent heat released during the condensation of saturated steam; after complete condensation, it is discharged through a drain valve! If superheated steam is used for heating, the temperature of the heat exchange medium can be increased, but it is necessary to increase the heat exchange area of the heat exchanger, reduce the temperature of the superheated steam to that of saturated steam, and then utilize the latent heat released during its condensation. As a result of this, the heat exchange area increases, the overall size of the heat exchanger grows, and the equipment investment rises ； Unless there are special requirements, saturated steam is generally used! There are three points to consider: 1 Saturation – the latent heat of vapor condensation is much greater than the sensible heat required to cool superheated vapor ； 2 The heat transfer film coefficient for the condensation of saturated steam (with phase change) is much larger than that for the cooling of superheated steam (without phase change) ； 3 When the superheated steam temperature is too high, it has a certain impact on the service life of the equipment.

After superheated steam enters the heat exchanger, although it is cooled by the cold medium, it still needs to be cooled further in order to achieve condensation; the heat transfer efficiency during this transition from a superheated state to a saturated state is very poor, with a K value of around 200. If saturated steam is used, the situation is different; when saturated steam is cooled, a liquid film forms on the heat transfer surface, resulting in a significant increase in the heat transfer coefficient. In the case of heat transfer between vapor and liquid, the K value can range from 600 to 1000 in practical applications. For horizontal condensation, it is better to use tubes at a 45-degree angle; however, the number of tubes per unit area is less when using this angle compared to 60 degrees.

Using saturated steam can reduce the fouling thermal resistance, thereby improving the service life of the equipment to a certain extent. Excessive use of hot steam causes the inlet flow rate of the material, the inlet temperatures of the heat exchangers at various temperature stages, and the fouling thermal resistance to deviate from the design conditions, which inevitably leads to the steam temperature inside the pipes deviating from the allowable values; therefore, using saturated steam for heat transfer results in greater stability. However, this also determines the structure of the device, which is more complex compared to other heat exchangers

Saturated steam is good because, when it is used for heat exchange, it utilizes the phase change heat (latent heat) directly. When superheated steam is used for heat exchange, it must first be cooled to its saturation temperature before its phase-change heat (latent heat) can be utilized; since the thermal content of the gas is low, this reduces the efficiency of the heat exchanger.

I also have a question related to coating chemistry: it’s difficult to meet the standards for impact resistance when using high-temperature painting (at 140 degrees Celsius). The standard requires that no cracks appear in the paint film when subjected to an impact force of 5 KG from a height of 50 cm; the impact tests from the front side succeed, but those from the back side fail. Why is that?