The seven stages of chemical engineering technology from concept to industrialization (Issue 38/100) -- Heat balance

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This post was last edited by xiouxingzhe on 2026-6-10 07:02. The seven stages of chemical technology from concept to industrialization (Issue 38/100) —— Technology refinement: Heat balance calculations. Dear friends: Hello everyone! In the previous issue, we discussed material balance calculations; now we know the flow rate, composition, temperature, and pressure of each stream in the entire process. In this issue, we move on to the second item of the “three balances” – heat balance calculation. Simply put, the questions that heat balance aims to answer are: where heating or cooling is required throughout the entire process, what the thermal load is, and what working fluid should be used for heat exchange. Once this work is completed, there will be a basis for the specifications and quantities of utility systems, and the heat load of the heat exchangers listed in the equipment data sheet will also have a source. I. From material balance to heat balance: The material balance sheet is the starting point for heat balance. The temperature, flow rate, specific heat, and phase state of each stream of fluid are the fundamental inputs for heat calculation. The first step in heat balance is to analyze each process point throughout the entire process that requires heating or cooling. Points that require heating — feed preheating, reboiler heating, reactor temperature rise. Points that require cooling – reactor heat removal, top of tower condensation, product cooling. For the analysis of each point, it is necessary to clarify: what temperature range needs to be achieved, and what the approximate amount of heating or cooling required is. Then, an appropriate utility medium is selected based on the different temperature ranges. Common heating media, from low to high temperature, are roughly hot water, low-pressure steam, medium-pressure steam, heat transfer oil, molten salt, or flue gas. Common cooling media, from high to low temperature, are roughly circulating water, chilled water, low-temperature refrigerants such as ethylene glycol aqueous solutions or Freon, and special media like liquid nitrogen for even lower temperatures. My principle is: use as few types of utility media as possible in a project. There is one or two hot working fluids, and one or two cold working fluids. For each additional medium chosen, there is an extra set of systems for storage, transportation, distribution, and recovery, as well as additional monitoring and security facilities; this increases the management costs over the long term. Therefore, when making a selection, it is preferable to use universal working fluids whenever possible; and if a low-temperature heat source is available, a high-temperature one should not be used. When selecting a model, the actual conditions at the location where the device will be installed must also be taken into account. The temperature difference that the local recirculating water can provide between summer and winter, whether the chilled water system needs to be protected from freezing during the coldest winter periods, as well as the pressure level and cost of the steam source—these factors are directly related to economic considerations. II. Heat exchanger network optimization: The basic concept of pinch analysis. It is easy to design individual heat exchangers, but difficult to optimize the heat exchanger network as a whole. The core idea of optimization is to prioritize direct heat exchange between hot and cold process streams, allowing the hot streams that need to be cooled to heat the cold streams that need to be warmed. This saves both heating utilities and cooling utilities, achieving two benefits at once. The systematic approach to this idea is the “pinch point technique”. It’s a bit complicated to explain, but the core logic involves these few steps. The first step is to identify from the entire process all cold streams that require heating and all hot streams that require cooling; for each stream, it is necessary to provide the initial temperature, target temperature, and heat capacity flow rate. In the second step, all cold streams are combined into one cold composite curve, and all hot streams are combined into one hot composite curve. In the third step, place the two curves on the same temperature graph, and move the cold curve back and forth until the minimum temperature difference between the two curves reaches the set minimum allowable heat transfer temperature difference; the position of this minimum temperature difference is the \"dead zone\". Step four: The minimum amount of heating utility required and the minimum amount of cooling utility required can be determined from the location of the pinch point – external heating is needed above the pinch point, while external cooling is needed below it. Pinching analysis also revealed several basic principles. Do not transfer heat across the pinch point – do not use cooling utilities above the pinch point, and do not use heating utilities below it; violating this principle will result in an equal increase in the consumption of utilities. No cooling utilities should be introduced above the pinch point, and the hot streams below the pinch point should be recycled as much as possible. Not every project requires rigorous pinch analysis. However, for projects with high demands on utility services – high steam consumption and high circulating water usage – pinch analysis can identify numerous energy-saving opportunities. Even without conducting a full analysis, adhering to principles such as “maximizing heat exchange between process streams, avoiding the use of high-temperature heat sources for low-temperature heating, and arranging heat exchanges in ascending order from low to high temperatures” when designing a heat exchanger network can help prevent much energy waste. III. Utility Consumption Table: Once the heat load at each heat exchange point and the utilities required are determined, they are compiled into a total consumption table. This consumption table must include at least: the type of working fluid used, the specifications of the working fluid, the device identifier for which the working fluid is used, the heat transfer amount handled by the working fluid, and the calculated amount of working fluid required. This table serves as the basic basis for subsequent handover to the civil engineering team. There is a common mistake here: calculating the consumption of utilities only based on normal operating conditions. In reality, driving conditions may require additional heat to raise the temperature and establish backflow, while stationary conditions may require additional cooling to safely lower the temperature. If the utility systems are designed only for normal operating conditions, they may not be able to meet the demand during startup. The peak consumption of utility services under different operating conditions determines the design capacity of the utility system. I have a deep understanding of this point. Especially when using pinch analysis for the optimization of heat exchange networks, it is essential to distinguish between different operating conditions – the pinch points under normal operating conditions may differ from those under startup conditions. A heat exchange network that is well-optimized for normal operating conditions may fail to achieve the designed heat exchange efficiency at startup due to the absence of a certain stream, thereby making startup difficult. This lesson came at a cost, and special attention must be paid to it in such projects. IV. Several common pitfalls in practice: There are several aspects that require special attention during the heat balance calculation. One is how to determine the margin for the heat load of the heat exchanger. Some people add 10% or 20% uniformly, which is not very reasonable. A reasonable approach is to analyze the sources of uncertainty in the heat load. The changes in heat load caused by fluctuations in material flow rate can be estimated from the operational flexibility range of material balance. The deviation in heat load caused by deviations in material property data can be estimated from the uncertainty of those material property data. The impact of scaling on the reduction in heat transfer efficiency can be informed by operational experience from similar installations. By adding up these uncertainties, a well-founded margin is determined, rather than arbitrarily assigning a percentage. Excessive margin affects both equipment investment and operational energy consumption ； There isn’t enough headroom; it will be more troublesome to make modifications after the device is put into operation. Another aspect that is easily overlooked is the peak heat load during intermittent operation. Some processes are operated intermittently—such as the heating phase of a reactor, where the steam demand can be several times higher than during the holding phase. The design of utility systems must be based on peak values, not average values. Attention should also be paid to the operating conditions of some special working fluids. For example, in a heat transfer oil system, the dehydration and heating process during the first operation takes a long time, and improper handling can lead to oil leaking from the expansion tank due to the vaporization of water. Sufficient operating margins and emergency plans must be reserved when designing utility systems. Preview for the next issue: Issue 39 – Momentum balance calculations: Determining the parameters of pumps and compressors. Once the heat balance is established, it becomes clear where heating and cooling are required. But the flow of materials through pipes and equipment is driven by a pressure difference. Which parts of the entire process require pumps and which require compressors, and how to calculate head and power—these are the issues that momentum balance is designed to address. To be continued in the next issue.