Difference in heat exchange between 8 kg of steam and 4 kg of steam
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Dear seniors, I have a question for everyone. 1. The steam used by our company is saturated steam from a centralized heating system; due to the long pipelines (about 10 kilometers), the steam delivered to the workshops has a high moisture content ； At the same time, the supply pressure fluctuates significantly; sometimes the pressure on the cylinder is 8 kilograms, while at other times it drops to as low as 4 kilograms. 2. The steam-powered equipment used by our company is primarily belt dryers – these devices condense steam into water through radiators, and the heat emitted by the radiators is transferred to the material via centrifugal fans to carry out the dehydration and drying process (it is generally required that the hot air blown by the centrifugal fans be at a temperature of 100–130 degrees), thereby taking advantage of the latent heat of vaporization of steam. 3. Under this process, when the steam pressure is 8 kilograms or 4 kilograms respectively, will the amount of reagents required increase to produce the same quantity of product? Will the production time increase?
Depending on the temperature range you need, 4 kilograms of steam can meet the requirements; it’s just that its heat transfer speed is not as fast as that of 8 kilograms of steam!
The production process works as follows: Generally, the temperature of the hot air in the first stage is set at 130 degrees. When the temperature reaches 130 degrees, the solenoid valve that supplies steam closes, stopping the steam supply; when the temperature drops below 130 degrees, the solenoid valve opens, and heating resumes, creating a cycle that repeats itself. When the product is dehydrated to a certain moisture content, it enters the second production stage, at which point the temperature of the hot air is reduced to 100 degrees. What I would like to ask now is: under such a production process, will the consumption volume be much higher when the steam supply pressure is 4 kilograms compared to when it is 8 kilograms? Will the production time increase?
Gases are different from liquids. There is a strict one-to-one relationship between steam temperature and pressure; different temperatures result in different heat transfer coefficients.
According to the table showing the relationship between steam temperature and pressure found online, when the steam pressure is 8 kilograms, the temperature is 170 degrees； When the steam pressure is 4 kilograms, the steam temperature is 144 degrees. The temperature of the hot air required for production is 100 degrees, so whether it is steam at a pressure of 8 kilograms or steam at a pressure of 4 kilograms, its temperature is sufficient to meet the 100-degree requirement for production. Meanwhile, checking the enthalpy values again: the enthalpy at a pressure of 8 kilograms is 2773.7 kg/kg, while it is 2742.1 kg/kg at 4 kilograms, with not much difference between them. What I want to know at the moment is that, with the same production equipment and the same process requirements, will the consumption increase when using 4 kilograms of steam compared to 8 kilograms? By how much would it increase if it were raised?
With the same heat exchange area, a higher pressure corresponds to a higher initial temperature; therefore, the logarithmic temperature difference will definitely be greater than before. Given that the amount of heat to be exchanged remains constant and the heat transfer coefficients are similar, the amount of steam required, which is 8 kilograms in this case, will definitely be less. The exact figure can be determined through detailed heat exchange calculations.
Hello! The radiators currently in use by our company utilize copper tubes as the flow channels, with copper fins attached to the outside for heat dissipation; their heat dissipation area is approximately 60 square units. When production is carried out at a normal steam pressure of 8 kilograms, assuming the steam consumption is 150 kg per hour, could you please advise me on what the approximate steam consumption would be at a steam pressure of 4 kilograms?
Your heat exchange equipment doesn’t have any data available, and conducting calculations for such equipment usually involves complex process calculations, which is rather troublesome. You can use Heat Exchanger Design Master or HITR for these calculations; you may want to consult Teacher Wiseboy for more details. @wiseboy
The gas flow rate must increase, and the time will also increase. The increase in gas flow rate can be determined from the enthalpy ratio, while the time is related to the temperature difference and the heat transfer area, making it difficult to calculate precisely.
At a steam pressure of 4 kilograms, the consumption increases and the production time lengthens.
This post was last edited by wiseboy on 2015-7-9 17:02, version 1. If the heat transfer area is sufficient, the steam demand changes very little, as the latent heat of vapor for both types remains relatively constant ； 2. If your original heat exchanger was properly designed, then after switching to low-temperature steam, the lower temperature difference may result in the same amount of steam not being able to condense. For example, when 150 kg of steam is introduced, it has already been condensed; but after switching to low-temperature steam, again 150 kg of steam is fed in, and 40 kg remains uncondensed – which is a waste. Additionally, the temperature rise on the cold side is also less than before. Therefore, the core of the problem is that the temperature difference for heat transfer varies greatly, while the heat content of the two types of steam remains relatively constant.