Talking about energy-saving design of circulating cooling water system in iron and steel enterprises

I. Overview

Each unit steel companies iron, steel making, casting, hot rolling, oxygen, and the like have a large power plant owned industrial cooling water users, industrial cooling water system as the primary production process lifeline for normal production And the safety of the maintenance equipment plays a vital role. The industrial circulating cooling water system has the characteristics of complex system, many users, large amount of water, and many types of circulating water media. The energy consumption of the circulating cooling water system is also extremely high. The electricity load accounts for about 20-30% of the electricity consumption of the entire unit project. The energy consumption of the circulating cooling water system in the large-scale steel combined enterprise unit can reach thousands or even tens of thousands of kilowatts. .

In the design process of industrial circulating cooling water system, implementing energy-saving measures, carrying out energy-saving design, and reducing the power consumption of the water system will help control the energy consumption of the entire project, and have positive significance for energy saving and emission reduction. This paper discusses how to carry out energy-saving design of industrial circulating cooling water system in steel enterprises, which can be used as reference and reference for practical engineering.

Second, the classification and composition of industrial circulating cooling water systems

According to the classification of circulating water media, the industrial circulating cooling water system of iron and steel enterprises can be generally divided into the following categories: pure water (or soft water) closed circulating water treatment system, open industrial net circulating water treatment system, and turbid circulating water treatment system.

(1) Pure water (or soft water) closed circulating water system

Pure water (or soft water) closed circulating water systems are often used for indirect cooling of critical equipment. The circulating water in the pure water (or soft water) closed circulating water system is basically isolated from the outside to ensure water quality.

The process flow of a commonly used pure water (or soft water) closed circulating water system is as follows:

Water treatment station circulating water supply pump water → process equipment → plate heat exchanger → return water to water treatment station circulating water supply pump;

Water treatment station circulating water supply pump water → process equipment → evaporative air cooler → return water to the water treatment station circulating water supply pump.

(2) Open industrial net circulating water system

Open industrial net circulating water systems are commonly used for indirect cooling of general equipment and as refrigerant water for heat exchangers. The industrial net circulating water system is an open system and the most common circulating water system.

The common process flow for industrial net circulating water systems is:

Water treatment station circulating water supply pump water → self-cleaning filter → process equipment → cooling tower → cold water pool → water treatment station circulating water supply pump water absorption;

Heat exchanger refrigerant water effluent (temperature rise) → cooling tower → cold water pool → heat exchanger circulating water supply pump → self-cleaning filter → heat exchanger refrigerant water into the water.

(3) Turbid circulating water system

Coal Gas turbid circulating water system used in the metallurgical industry, iron making, steel making, casting, hot rolling and the like of the cleaning unit, slag, flame cutting, spray cooling, quenching, refining dust and the like.

The common process flow of the turbid circulating water system is as follows.

Gas cleaning turbid circulating water: wastewater → elevated trough → coarse particle separator → radial flow sedimentation tank → hot water pool → cooling tower → cold water pool → return water to the user;

Slag slag circulating water: slag water → iron skin ditch → swirling pool → advection sedimentation tank (with oil sling device) → high-speed filter → cooling tower → cold water pool → return water to the user;

Quenching and cooling turbid circulating water: user return water → iron skin ditch → hot water pool → partial water filtration → partial water cooling tower → cold water pool → mixed with uncooled, filtered water → return water to the user;

Flame cutting and spray cooling turbid circulating water: user return water → tin skin ditch → swirling pool → advection sedimentation tank (with oil skimming device) or chemical degreasing precipitator → hot water pool → cooling tower → cold water pool → high speed filter → back Water to the user;

Refining dust and turbid circulating water: user return water → hot well tank or hot water pool → mixing pool → reaction tank → advection sedimentation tank (or sloping sedimentation tank) → hot water pool → cooling tower → cold water pool → return water to users

Third, the composition of industrial circulating cooling water system energy consumption

From the above analysis of various industrial circulating cooling water system processes, the energy consumption of the industrial circulating cooling water system is composed of the following parts: water consumption, water pressure energy consumption, water consumption energy consumption, heat energy consumption, distance energy consumption and Hydraulic imbalance energy consumption, etc.

(1) Water consumption and energy consumption

Regardless of the closed circulating water system, the open industrial net circulating water system or the turbid circulating water system, the process equipment users need a large amount of circulating cooling water, and the water supply must be supplied with power. If there are many users and the water volume is large, the demand for electricity is large. It also means that the energy consumption is high, the support is small, and the small amount of water is small, which means that the energy consumption is small.

(2) Water pressure energy consumption

Closed circulating water system, open industrial net circulating water system or turbid circulating water system, different process equipment users, their water pressure requirements are also different, high pressure requirements, high energy consumption, low pressure requirements, low energy consumption.

In terms of water pressure energy consumption, the water pressure to be added to the closed circulating water system is used to compensate for the head loss of the entire pipe network system (including the head loss of equipment, pipelines, valves, etc.); and the open industrial net circulating water system There is a pressure relief point in the turbid circulating water system, and the water supply pressure is determined according to the user's requirements and the head loss of the pipeline along the path.

Although the open industrial net circulating water system is a closed pipe network in the workshop process equipment user, there is a cooling tower at the water treatment station, and the cooling tower is the pressure relief point of the open industrial net circulating water system. The turbid circulating water system is usually the pressure relief point at the process equipment, because the turbid circulating water is generally used to directly cool the process equipment or flushing, and must be relieved.

Therefore, in terms of water pressure energy consumption, the water pressure energy consumption of the closed circulating water system is much lower than that of the open industrial net cycle due to the existence of pressure relief and leakage in the open industrial net circulating water system and the turbid circulating water system. Water system and turbid circulating water system.

In addition, when considering the water pressure energy consumption, it is necessary to consider not only the factors of water supply pressure, but also the factors of pressure return. For the circulating water system, there is a return water when there is water supply. The return water pressure must be sufficient to return from the main process unit workshop to the circulating water treatment station.

(3) Energy consumption by water

The energy consumption by water is mainly reflected in the water system. The water use system is divided into a continuous water system and a discontinuous water system. The electricity consumption of the continuous water system must be higher than the intermittent water system, and the energy consumption of the continuous water system must also be higher than the energy consumption of the intermittent water system.

(4) Thermal energy consumption

Industrial circulating cooling water, especially closed circulating cooling water systems and open industrial net circulating water systems, whose main function is to take away a large amount of heat and cooling equipment generated by process equipment in the production process. For industrial circulating cooling water systems, the main way to remove heat is heat exchangers, evaporative air coolers or cooling towers. If a heat exchanger is used as a means of indirect cooling, the cooling tower is ultimately cooled.

Cooling towers and evaporative air coolers must exchange heat between the circulating water system and the atmosphere, which inevitably consumes electricity and consumes energy. The greater the temperature difference between the circulating water system and the return water, the greater the heat that needs to be exchanged, and the greater the power consumption of the fan, the higher the energy consumption. In addition, if the water temperature required by the user is lower, it will directly lead to an increase in the power consumption of the cooling tower or the evaporative air cooler and an increase in energy consumption.

(5) Distance energy consumption

The distance energy consumption mainly refers to the head loss caused by the pipeline distance between the water treatment station and the main process unit workshop. The water supply return pipe is short, the head loss of the pipe is small, the motor power of the water supply pump can be appropriately reduced, and energy can be saved in long-term operation. Take an industrial net circulating water treatment station with a circulating water volume of 10000 m ³ /h as an example. For every 100m of supply and return water pipeline, about 40kW can be saved.

(6) Hydraulic imbalance energy consumption

The industrial circulating cooling water system of iron and steel enterprises is complex, the water volume is large, the users are many and scattered, the user position is high and the water consumption is often different. During the commissioning and production operation, the hydraulic imbalance between users occurs from time to time. . In some process equipment, the water consumption and water supply pressure requirements are always too small to be met. Usually, the following measures are taken: 1) Add a new pressurized pumping station to increase the water pressure before the local user, if pumping directly from the pipe network The requirement is that the diameter of the circulating water main pipe to be pumped is large enough to not affect the water consumption of nearby users. If the water tank is added, it will increase the additional kinetic energy and increase the investment of civil works; 2 increase the total circulating water volume in the whole circulating water system and Water pressure. But these are just masking the hydraulic imbalance and have not really solved the problem. Some process equipment uses too much water head left in the water point. In the case of a large amount of water discharge or overpressure, only decompression and throttling measures can be added on the pipe network. The hydraulic imbalance not only causes confusion inside the circulating water system, but also affects production, and also wastes a lot of energy. This is the so-called hydraulic imbalance energy consumption.

Fourth, energy-saving design measures

To implement the energy saving of the circulating cooling water system, it is necessary to analyze the energy consumption composition of the above industrial circulating cooling water system, and propose reasonable energy saving suggestions and measures for various energy consumption, as detailed below.

(1) Water consumption and energy consumption

Usually, the circulating water volume of the industrial circulating cooling water system of the iron and steel enterprise is obtained by summing the amount of water required by each process equipment. On the surface, the amount of water seems to be certain and unique, but in fact it is completely possible to control the amount of circulating water and reduce the energy consumption of water through reasonable technical analysis.

First of all, for the users of various process equipment, the main purpose of circulating water is to take away heat and cool the equipment. The main indicator controlled by each process equipment support is also the temperature, especially the inlet water temperature. Set the heat to be taken away by the circulating cooling water system to W, the inlet water temperature is t1, the outlet water temperature is t2, the circulating water quantity is Q, and the specific heat of water is γ, then W=Q·γ·(t2-t1 ). The heat W is generated during the production process of the process equipment, and is generally a certain value. The inlet water temperature is also determined due to stricter limits. If the water temperature of the effluent can be increased, the amount of circulating water can be reduced, thereby achieving the purpose of reducing the energy consumption of the water. If the temperature difference between the inlet and outlet water of the process equipment user is increased from 10 ° C to 15 ° C, the circulating water volume can be reduced by 1/3.

Of course, increasing the effluent temperature of the process equipment will inevitably increase the inlet water temperature of the subsequent cooling tower or the evaporative air cooler, resulting in an increase in heat energy consumption, which will be discussed later.

(2) Water pressure energy consumption

Water pressure energy consumption includes water supply water pressure energy consumption and return water pressure energy consumption. The type of general circulating water system is determined according to the importance of the user and the user's requirements for water quality. The water supply pressure is also determined according to the needs of the user. Therefore, the most critical treatment to reduce water pressure energy consumption is how to deal with the problem of backwater pressure and energy consumption.

For the closed circulating water system and the open industrial net circulating water system, the return water pressure, that is, the water supply pressure minus the residual pressure after the head loss of the process equipment, as long as the calculation is accurate, the reasonable water supply pressure is determined to determine the return water. The pressure is gone. But for turbid circulating water systems, the situation is more complicated. Turbid circulating water is mainly used for direct cooling or flushing of equipment. After the user, it is usually relieved. Therefore, to control the water pressure of the return water pressure, in fact, it is necessary to implement gravity flow back to the water.

To carry out gravity flow back to the water, it is first necessary to judge whether the elevation of the two processes can meet the most basic requirements of gravity return water from the main process equipment to the circulating water treatment facility. Secondly, it is necessary to conduct a thorough analysis of the site situation or the professional construction drawings to find a suitable and reasonable route. At present, Baosteel has successfully implemented the RH turbidity cycle without pressure gravity flow backwater, which has changed the previous RH turbidity cycle backwater using hot well pump to improve the delivery to water treatment facilities, which has achieved good economic results and greatly reduced the RH energy consumption.

(3) Energy consumption by water

At present, most of the users in the actual production process are intermittent water, and from the perspective of safe production, the circulating cooling water system of the iron and steel enterprises is considered according to the continuous water consumption of each user from the design point of view and the production operation angle. The circulating cooling water system is a constant pressure water supply system. The conventional method is to install a bypass valve or a pressure relief valve on the circulating water main pipe. When the user's water demand is reduced or the number of users is reduced, the pipe network pressure will be caused. The rise, when the bypass or pressure relief valve is opened, still allows a constant flow to return to the water treatment station through the circulating cooling water system. This practice has caused a lot of energy waste. Therefore, how to reduce the energy consumption caused by water use must be studied.

With the development of domestic technology, domestic frequency conversion technology has become increasingly mature and reliable. It is a reliable energy-saving means to set up a variable frequency water pump set or set up one or two frequency conversion water pumps in a group of water pumps.

The calculation formula in the supervision guide of the mandatory national standard implementation of GB12497 "Economic operation of three-phase asynchronous motor", Ki is the power saving rate after speed regulation, QN is the rated flow rate, Q is the actual flow rate, if the flow rate adjustment range is Q= (0.5~1) QN, the power saving rate after speed regulation is Ki=1-(Q/QN)3/[0.45+0.55(Q/QN)2], and the power saving rate is up to 78.7%.

(4) Thermal energy consumption

The heat energy consumption is mainly reflected in the kinetic energy consumed by the cooling tower or the evaporative air cooler in cooling the industrial water in the circulating water system.

The heat to be carried away by the cooling tower or evaporative air cooler is the amount of heat transferred from the equipment to the circulating water during the production process. The amount of heat that the cooling tower or evaporative air cooler takes away depends on how much heat is transferred from the equipment to the circulating water. The amount of this heat determines the kinetic energy that must be consumed by the cooling tower or evaporative air cooler.

However, the value of the kinetic energy required to determine the cooling tower or evaporative air cooler, in addition to the heat generated by the process equipment during the production process, the outlet water temperature of the cooling tower is the value required to be cooled to a certain temperature index. Played a big role. For example, taking away the same amount of heat, the energy required to reduce circulating water from 50 ° C to 40 ° C is much less than the energy that reduces circulating water from 43 ° C to 33 ° C. Under the premise of the conditions of the process equipment, the water supply temperature of the circulating cooling water, that is, the outlet water temperature of the cooling tower or the evaporative air cooler should be increased as much as possible.

In the previous water consumption energy consumption, it is mentioned that increasing the effluent water temperature of the process equipment can reduce the amount of circulating water and water, thereby achieving the purpose of reducing the energy consumption of the water. However, the effluent temperature of the process equipment is bound to increase the inlet water temperature of the subsequent cooling tower or the evaporative air cooler, resulting in an increase in heat energy consumption. From the engineering point of view, increasing the heat energy consumption of the effluent water temperature of the process equipment is much less than the energy consumption saved by reducing the circulating water volume. Therefore, it is still an effective energy-saving measure to increase the effluent water temperature of the process equipment within the scope of the process.

In addition, in the current actual engineering design process, the most unfavorable temperature or temperature difference of the process equipment is often used as the basis for the selection of the cooling tower or the evaporative air cooler. In fact, there are a large number of circulating water users in the circulating water system, and some users have large temperature difference for returning water, and some users have small temperature difference for returning water. Therefore, it is recommended to use the weighted average design method to calculate the temperature difference between the supply and return water of the integrated circulating cooling water system to select the cooling tower. In the calculation of the temperature and heat of the circulating water system, more heat-balanced design methods are used instead of simple or mechanical collection of process equipment data, which reduces the unnecessary energy consumption due to excessively rich calculations. Extremely important role.

(5) Distance energy consumption

Where conditions permit, the location of the industrial circulating cooling water treatment station should first be placed as close as possible to the main process and close to the main user.

(6) Hydraulic imbalance energy consumption

Hydraulic imbalance is usually divided into four situations: 1 due to unreasonable design, users with widely different water pressure requirements are placed in a circulating water system, artificially causing hydraulic imbalance within the system; 2 due to pipe network hydraulic calculation Unreasonable, improper selection of pipe diameters, resulting in a large difference in head loss between different users in the same circulating water system, resulting in hydraulic imbalance; 3 industrial circulating water users themselves may be changing, water consumption The change causes the pipeline performance curve and working point to change constantly, which causes the flow and head of the pump of the circulating water system to change, which causes the hydraulic imbalance of the system. This situation is particularly obvious for the function and impact of the closed circulating cooling water system; The imbalance caused by the limitations of each user in the same system, such as the water supply pressure of the user near or the lower position of the pump is high, the relative flow is large, and the water supply pressure is lower than the flow rate of the user who is far away from the pump or the position is low. The amount of water and water pressure required for the production of process equipment users have certain requirements, resulting in this

The hydraulic imbalance energy consumption is essentially a waste of energy caused by the unreasonable or imperfect of the circulating water system. Implementing the hydraulic balance design during system design is the only way and measure to reduce the energy imbalance of hydraulic imbalance. The details are as follows:

1 It is required to minimize the quantity of industrial circulating cooling water system in the project through reasonable system division and merging, so as to save one-time project cost and investment; 2 through more accurate hydraulic calculation, reasonably select the diameter of circulating water pipeline to avoid circulation The water pipe network resistance is too large or too small; 3 take appropriate water volume and water pressure adjustment measures, so that different users in the same circulating water system are less affected by external factors such as distance from water source or user position, etc. The influence between each other is also minimized; 4 through the reasonable selection of the water source pump, the water supply source has a certain adjustment capacity as much as possible, in line with the production practice.

Through the above work, the requirements of the water consumption and water supply pressure of each user inside the circulating water system can be satisfied, and the energy saving requirements must be met.

V. Summary

The energy-saving design of industrial circulating cooling water systems in steel companies is a system design that requires analysis, calculation and continuous optimization of various energy consumption links.

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