Working principle and cycle

The vapor pressure of the solution is relative to the equilibrium state. If the lithium bromide solution with a vapor pressure of 0.85kPa comes into contact with water vapor with a pressure of 1kPa (7℃), the vapor and liquid are unbalanced, and the solution has the ability to absorb water vapor until the water vapor pressure is slightly higher than 0.85kPa (for example, 0.87kPa).

Figure 1 absorption refrigeration principle

The pressure difference between 0.87kPa and 0.85kPa is used to overcome the flow resistance in the connecting pipeline and the pressure difference generated when the process deviates from the equilibrium state, as shown in figure 1. When water evaporates at 5℃, it is possible to absorb the latent heat of vaporization from the cooled medium with high temperature and cool the cooled medium.

In order to make water vaporize continuously at low pressure and the generated steam be absorbed continuously, so as to ensure the continuous absorption process, the concentration of the absorption solution must be greater than that of the absorbed solution. Therefore, in addition to continuously supplying pure water to the evaporator, new concentrated solution must be continuously supplied, as shown in figure 1. Obviously, it is not economical to do so.

Figure 2 Single-effect lithium bromide absorption refrigerator system Figure 3 Double-tube lithium bromide absorption refrigerator system

1- condenser; 2- generator; 3- evaporator; 4- absorber; 5- heat exchanger; 6-U tube;

7- Anti-crystallization tube; 8- air extraction device; 9- Evaporator pump; 10- absorption pump; 1 1- generator pump; 12- three-way valve

In fact, the dilute solution is heated to boiling, so that the distilled water obtained continues to evaporate, as shown in Figure 2. The system consists of generator, condenser, evaporator, throttle valve, pump and solution heat exchanger. Before heating, use a pump to increase the pressure of the dilute solution, so that the steam generated by boiling can be condensed at room temperature. For example, when the cooling water temperature is 35℃, considering the allowable temperature difference of heat transfer in the heat exchanger, condensation may occur around 40℃, so the pressure in the generator must be 7.37kPa or higher (considering factors such as pipeline resistance).

The pressure difference between the generator and condenser (high pressure side) and the evaporator and absorber (low pressure side) is maintained by expansion valves or other throttling mechanisms installed on corresponding pipelines. In lithium bromide absorption refrigerator, this pressure difference is quite small, generally only 6.5~8kPa, so U-tube, throttle short tube or throttle orifice plate can be used.

The temperature of concentrated solution leaving the generator is very high, while the temperature of dilute solution leaving the absorber is very low. It is impossible for the concentrated solution to absorb water vapor until it is cooled to the temperature corresponding to the absorber pressure, and the dilute solution must be heated to the saturation temperature corresponding to the generator pressure to start boiling. Therefore, before the concentrated solution and the dilute solution enter the absorber and the generator respectively, the solution heat exchanger is used to exchange heat between the concentrated solution and the dilute solution, so that the temperature of the dilute solution increases and the temperature of the concentrated solution decreases.

Because the specific volume of water vapor is very large, thick pipes are needed to avoid excessive pressure drop when flowing. To avoid this situation, the condenser and generator are usually placed in one container, and the absorber and evaporator are placed in another container, as shown in Figure 3. It is also possible to put these four main devices in a shell, and the high-pressure side and the low-pressure side are separated by a partition, as shown in Figure 4.

Fig. 4 Single barrel lithium bromide absorption chiller system

1- condenser; 2- generator; 3- evaporator; 4- absorber;

5- heat exchanger; 6, 7, 8- pumps; 9-u tube

To sum up, the working process of lithium bromide absorption refrigerator can be divided into two parts:

The refrigerant vapor generated in the (1) generator condenses into refrigerant water in the condenser, enters the evaporator through the U-shaped tube, and evaporates at low pressure, resulting in refrigeration. These processes are exactly the same as those of vapor compression refrigeration cycle in condenser, throttle valve and evaporator.

(2) The concentrated solution flowing from the generator enters the absorber after depressurization, absorbs the refrigerant vapor generated by the evaporator to form a dilute solution, which is pumped to the generator and then heated to form a concentrated solution. The functions of these processes are equivalent to those of the compressor in the vapor compression refrigeration cycle.

Representation of working process on diagram

The ideal working process of lithium bromide absorption refrigerator can be represented by chart, as shown in Figure 5. The ideal process means that the working medium does not have any resistance loss in the flow process, the equipment does not exchange heat with the surrounding air, and the solution reaches equilibrium at the end of generation and absorption.

Fig. 5 The working process of lithium bromide absorption refrigerator is shown in the figure.

(1) occurrence process

Point 2 represents the saturated dilute solution state of the absorber, with concentration of, pressure of and temperature of. After passing through the generator pump, the pressure is increased to and then sent to the solution heat exchanger. Under the condition of constant pressure, the temperature rises from to, and the concentration remains unchanged, and then it enters the generator and is heated by working steam in the heat transfer tube of the generator. The temperature rises from the saturation temperature under pressure and begins to boil under constant pressure. The water in the solution evaporates continuously, the concentration gradually increases, and the temperature also increases. 2-7 indicates the temperature rising process of the dilute solution in the solution heat exchanger, and 7-5-4 indicates the heating and generating process of the dilute solution in the generator. The generated water vapor state is represented by the average state point 3' of the starting state (point 4') and the ending state (point 3'). Because pure water vapor is produced, the state is located on the longitudinal axis of.

(2) condensation process

The steam generated by the generator (point 3') enters the condenser and is cooled by the cooling water flowing in the condenser tube under constant pressure. It first becomes saturated vapor and then condenses into saturated liquid (point 3). 3'-3 represents the cooling and condensation process of refrigerant vapor in the condenser.

(3) Throttling process

Saturated refrigerant water (point 3) with pressure (=) passes through a throttling device (such as a U-tube) and then enters the evaporator after the pressure drops to (=). Because the enthalpy and concentration of cooling water are unchanged before and after throttling, the state point after throttling (not marked in the figure) coincides with point 3. However, due to the pressure drop, part of the refrigerant water is gasified into refrigerant vapor (point 1'), and most of the non-gasified refrigerant water is cooled to the saturation temperature corresponding to the evaporation pressure (point 1) and accumulated in the evaporator water pan, so point 3 before throttling represents the saturated water state at the condensation pressure, and point 3 after throttling represents the saturated vapor (point) and saturated liquid (point) at the pressure.

(4) Evaporation process

The cooling water (point 1) accumulated in the evaporator water pan is evenly sprayed on the outer surface of the evaporator tube cluster by the evaporator pump, which absorbs the heat of the cooling water in the tubes and evaporates, so that the cooling water changes from point 1 to 1' under the constant pressure and isothermal conditions, and 1- 1' represents the gasification process of the cooling water in the evaporator.

(5) absorption process

The solution with concentration of, temperature of and pressure of flows from the generator to the solution heat exchanger under the action of its own pressure and pressure difference, and transfers some heat to the dilute solution, and the temperature drops to (point 8). Figure 4-8 shows the exothermic process of concentrated solution in solution heat exchanger. The concentrated solution at state point 8 enters the absorber, where it is mixed with part of the dilute solution (point 2) to form an intermediate solution (point 9') with concentration and temperature, which is then uniformly sprayed on the outer surface of the absorber tube bundle by the absorber pump. After the intermediate solution enters the absorber, due to the sudden drop of pressure, a part of water vapor first flashes out and the concentration increases, as shown in point 9. Because the cooling water flowing in the absorber tube cluster continuously takes away the absorption heat released in the absorption process, the intermediate solution has the ability to continuously absorb the water vapor from the evaporator, which reduces the solution concentration and the temperature from (point 2). 8-9' and 2-9' represent mixing process, and 9-2 represents absorption process in absorber.

It is assumed that the flow rate of dilute solution sent to the generator is 0 and the concentration is 0, resulting in refrigerant vapor, and the remaining concentrated solution flows out of the generator with the flow rate and concentration. According to the mass balance relationship in the generator, the following equation is obtained.

Order, and then (1)

A is called the period ratio. Indicates the circulating amount of dilute lithium bromide solution required for each generation of 1kg water vapor in the generator. () is called deflation interval.

The process analyzed above is ideal. In fact, due to the existence of flow resistance, the pressure of water vapor drops when it passes through the water baffle, so in the generator, the pressure generated is greater than the condensation pressure, and the solution concentration will be reduced under the condition of constant heating temperature. In addition, due to the influence of the solution column, the solution at the bottom occurs under high pressure, and at the same time, due to the limited contact area and contact time between the solution and the surface of the heating tube, the concentration of concentrated solution at the end of the occurrence is lower than the ideal concentration, which is called (-) insufficient occurrence; In the absorber, the absorber pressure should be less than the evaporation pressure, which will increase the concentration of dilute solution under the condition of constant cooling water temperature. Due to the short contact time and limited contact area between the absorbent and the absorbed steam, and the existence of non-condensable gases such as air in the system, the absorption effect of the solution is reduced, and the concentration of the dilute solution at the end of absorption is higher than the ideal situation, which is called (-) insufficient absorption. Both insufficient occurrence and insufficient absorption will cause the change of parameters in the working process, which will reduce the deflation range and thus affect the economy of the cycle.

Thermodynamics and heat transfer calculation of lithium bromide absorption refrigerator

The calculation of lithium bromide absorption refrigerator should include thermal calculation, heat transfer calculation, structural design calculation and strength check calculation. Only the methods and steps of thermodynamic calculation and heat transfer calculation are described here.

Heat calculation

The thermodynamic calculation of lithium bromide absorption refrigerator is to reasonably select some design parameters (heat transfer temperature difference, air release range, etc.). According to the user's requirements for cooling capacity and cooling water temperature, as well as the conditions of heat source and cooling medium that users can provide, cyclic calculation is carried out again, which provides calculation and design basis for heat transfer calculation.

(1) known parameters

(1) Refrigeration capacity is put forward according to the requirements of production technology or air conditioning, taking into account cooling loss, manufacturing conditions and operation economy.

(2) The outlet temperature of chilled water is proposed according to the production process or air conditioning requirements. Because it is related to evaporation temperature. If it decreases, the refrigeration and thermal coefficients of the unit will decrease, so on the basis of meeting the requirements of production process or air conditioning, the evaporation temperature should be increased as much as possible. For lithium bromide absorption refrigerator, because water is used as refrigerant, it is generally higher than 5℃.

③ The inlet temperature of cooling water is determined according to local natural conditions. It should be pointed out that although reduction can reduce the condensation pressure and enhance the absorption effect, considering the special problems of lithium bromide crystallization, the lower the reduction, the better, but there is a certain reasonable range. When the unit is running in winter, it is especially necessary to prevent the cooling water temperature from being too low.

④ Considering the utilization, crystallization and corrosion of waste heat, it is reasonable to use 0. 1~0.25Mpa saturated steam or hot water above 75℃ as heat source. If higher steam pressure can be provided, the thermal efficiency can be further improved.

(2) Selection of design parameters

(1) absorber outlet cooling water temperature is 1, condenser outlet cooling water temperature is 2. Because the absorption refrigerator uses heat energy as compensation means, the heat taken away by cooling water is much larger than that of vapor compression refrigerator. In order to save the consumption of cooling water, cooling water often flows through the absorber and condenser in series. Considering the absorption effect in the absorber and the high condensation pressure allowed by the condenser, cooling water usually passes through the absorber first and then enters the condenser. The total temperature rise of cooling water generally needs 7~9℃, depending on the inlet temperature of cooling water. Considering that the heat load of the absorber is greater than that of the condenser, 1 the temperature rise through the absorber is higher than that through the condenser. Total temperature rise of cooling water. If the water source is sufficient or the heating temperature is too low, the cooling water can flow through the absorber and the condenser in parallel, and the temperature rise of the cooling water in the condenser can be higher. When the series connection mode is adopted,

(2)

(3)

(2) The condensation temperature and pressure are generally 2~5℃ higher than the cooling water outlet temperature, that is

(4)

According to the water vapor meter, namely

③ Evaporation temperature and pressure evaporation temperature are generally 2~4℃ lower than cooling water outlet temperature. If the requirements are low, the temperature difference is small, otherwise, the temperature difference is large, that is,

(5)

The evaporation pressure is obtained according to the following formula, namely

④ Minimum temperature of dilute solution in absorber The outlet temperature of dilute solution in absorber is generally 3~5℃ higher than that of cooling water, and a smaller value is beneficial to absorption effect, but the decrease of heat transfer temperature difference will lead to the increase of required heat transfer area, and vice versa.

(6)

⑤ Absorber pressure is lower than evaporation pressure due to the resistance loss when steam flows through the water baffle. The pressure drop is related to the structure of the water baffle and the airflow speed, and is generally taken as

(7)

⑥ The concentration of dilute solution is determined by the lithium bromide solution diagram according to the sum, namely

(8)

⑦ Concentration of concentrated solution In order to ensure the economy, safety and feasibility of circulation, it is hoped that the degassing range (-) of circulation will be between 0.03 and 0.06, so

(9)

Today, the maximum temperature of the solution in the generator The temperature of the concentrated solution at the generator outlet can be determined according to the following formula.

( 10)

The relationship between them is determined in the diagram of lithium bromide solution. Although there is resistance when the generated refrigerant vapor flows through the water baffle, its value is very small compared with that of, which can be ignored, so the assumption of = has little influence. It is generally expected to be lower than the heating temperature 10~40℃. If it exceeds this range, the relevant parameters should be adjusted accordingly. When the temperature is high, the temperature difference is larger.

Pet-name ruby solution heat exchanger outlet temperature and concentrated solution outlet temperature are determined by the temperature difference at the cold end of the heat exchanger. If the temperature difference is small, although the thermal efficiency is high, the required heat transfer area will still be large. In order to prevent the concentrated solution from crystallizing, it should be higher than the crystallization temperature corresponding to the concentration by more than 10℃, so the temperature difference at the cold end is 15~25℃, that is

( 1 1)

If the heat exchange between solution and environmental medium is neglected, the outlet temperature of dilute solution can be determined according to the heat balance formula of solution exchange, that is

( 12)

Is determined by and in the figure, where.

Attending the absorption tower spray solution state is to strengthen the absorption process of the absorption tower, and the absorption tower usually adopts spray form. Due to the small amount of concentrated solution entering the absorber, in order to ensure a certain spraying density, a certain amount of dilute solution is often added to form an intermediate solution and then sprayed. Although the concentration is reduced, the absorption effect is enhanced due to the increase of spraying amount.

Assuming that dilute solution is added to the concentrated solution, an intermediate solution with a state of 9' is formed according to the heat balance equation, as shown in Figure 6.

Let's order then

( 13)

F is called the dilute solution recirculation rate of the absorber. Its significance lies in that to absorb 1kg of refrigerant water vapor needs to supplement kg of dilute solution. Generally, sometimes the concentrated solution is sprayed directly, that is. Similarly, the concentration of the intermediate solution can be obtained from the mass balance equation of the mixed solution. that is

( 14)

Then, the temperature of the mixed solution is determined by the sum diagram.

(3) Calculation of equipment heat load

Calculate the heat load of the equipment according to the heat balance formula of the equipment.

① The flow rate of refrigerant water in the refrigerator is determined by the known refrigerating capacity and the unit heat load in the evaporator.

( 15)

As can be seen from fig. 7

( 16)

② The thermal load of the generator is shown in Figure 8.

that is

( 17)

③ The heat load of condenser is shown in Figure 9.

( 18)

④ See figure 10 for the heat load of absorber.

( 19)

⑤ See figure 1 1 for the heat load of solution heat exchange.

(20)

(4) Thermal balance, thermal coefficient and thermal perfection of the device

If the heat brought by the power consumed by the pump and the heat exchange between the system and the surrounding environment are ignored, the heat balance formula of the whole device should be

(2 1)

The thermodynamic coefficient is expressed by, which reflects the refrigeration capacity obtained by consuming unit steam heating and is used to evaluate the economy of the device. By definition,

(22)

The single-effect lithium bromide absorption refrigerator is generally 0.65~0.75, and the double-effect lithium bromide absorption refrigerator is usually above 1.0.

Thermal perfection is the ratio of thermal coefficient to the highest thermal coefficient at the same heat source temperature. Assuming that the heat source temperature is, the ambient temperature is, and the cold source temperature is, the maximum thermodynamic coefficient is

(23)

Thermal perfection can be expressed as

(24)

It reflects the irreversible degree of refrigeration cycle.

(5) Calculation of heating steam consumption and flow of various pumps.

① Consumption of heating steam

(25)

Where a- additional coefficient considering heat loss, a =1.05 ~1.10;

-Enthalpy of heating steam, kj/kg;

-Enthalpy of heating steam condensate, kJ/kg.

② The flow rate of absorption pump

(26)

Where-the amount of solution sprayed by the absorber, kg/s;

—— density of spray solution, kg/l, as shown in the figure.

③ Flow rate of generator pump

(27)

Where the density of dilute solution, kg/l, is obtained from the figure.

④ Flow rate of refrigerant water pump

(28)

Where-specific heat capacity of refrigerant water;

—— inlet temperature of cooling water,℃;

—— outlet temperature of cooling water,℃.

⑤ Flow rate of cooling water pump If cooling water flows through absorber and condenser in series, its flow rate should be determined from two aspects.

For the absorber

(29)

Used for condenser

(30)

The calculation result should be that if there is a big difference between the two, it means that the previously assumed total temperature rise distribution of cooling water is not suitable and needs to be re-assumed until the two are equal.

⑥ Flow rate of evaporator pump Because the pressure in the evaporator is very low, the hydrostatic pressure of refrigerant has a great influence on the evaporation boiling process, so the evaporator is made into a spray type. In order to ensure a certain spraying density and make the cooling water uniformly wet the outer surface of the generator tube bundle, the spraying amount of the evaporator pump is greater than the evaporation amount of the evaporator, and the ratio of the two is called the recirculation rate of the evaporator cooling water, which is denoted by a, and a= 10~20. The flow rate of evaporation pump is

(3 1)

Heat transfer calculation

(1) heat transfer calculation formula

The simplified heat transfer calculation formula of lithium bromide absorption refrigerator is as follows:

(32)

Where-heat transfer area,;

-heat transfer, w;

-the maximum temperature difference in the heat exchanger, that is, the difference between the inlet temperature of hot fluid and the inlet temperature of cold fluid,℃;

-a, b- constants are related to the way of fluid flow in the heat exchanger. See table1for specific data;

—— Temperature change of fluid A during heat exchange,℃;

—— Temperature change of fluid B during heat exchange,℃.

When Formula (32) is adopted, it is required that

If the concentration of the fluid changes during heat exchange, such as condensation in a condenser, since the temperature of the fluid does not change at this time, Equation (32) can be simplified as follows.

(33)

(2) Calculation of heat transfer area of various heat exchange equipment.

(1) the heat transfer area of the generator The dilute solution entering the generator is in supercooled state (point 7) and must be heated to saturated state before boiling (point 5). Because the heat required for heating up is very small compared with the heat required for boiling process, it is calculated according to the saturation temperature in heat transfer calculation. In addition, if the heating medium is superheated steam, the heat released from the superheated zone is far less than the latent heat, which is also calculated according to the saturation temperature. Due to the phase change in the process of heating steam heat exchange, the heat transfer area of the corresponding generator is

(34)

Where-heat transfer coefficient of generator,

(2) The heat transfer area of the condenser The refrigerant vapor entering the condenser is superheated steam. Because the heat released when it is cooled into saturated steam is far less than that released when it is condensed, it is still calculated according to the saturated condensation temperature. Because of the phase change of refrigerant water vapor during heat exchange, that is,

(35)

Where-heat transfer coefficient of condenser,

(3) Heat transfer area of absorber If the cooling water in the absorber is mixed and the spray liquid is not mixed, then

(36)

Where-heat transfer coefficient of absorber.

(4) Heat transfer area of evaporator. In the process of evaporation, the cooling water changes phase, and then

(37)

Where-heat transfer coefficient of evaporator.

⑤ The solution heat exchanger has large water equivalent due to the large flow of dilute solution and large heat transfer area, which should be the temperature change of dilute solution in the heat exchanger. The flow mode of two solutions in the heat exchange process is often in the form of countercurrent, then

(38)

Where-heat exchange and heat transfer coefficient of solution,

(3) Heat transfer coefficient

In the formula for calculating the heat transfer area of the above equipment, all parameters except the heat transfer number have been determined in the thermal calculation. Therefore, the essence of heat transfer calculation is how to determine the heat transfer coefficient K. Because there are many factors that affect the value of K, the test data of the same type of machines are often used as the basis for selecting the value of K in design calculation. Table 2 lists the heat transfer coefficients of some domestic and foreign products for design reference.

As can be seen from Table 2, the heat transfer coefficient of each equipment is quite different. In fact, factors such as heat flux density, flow velocity, spray density, material, pipeline layout, water quality, amount of non-condensable gas and dirt will all affect the value of heat transfer coefficient. At present, some improvement measures have been taken for lithium bromide absorption refrigeration units at home and abroad, such as properly handling heat transfer tubes, increasing water velocity and improving nozzle structure. And the heat transfer coefficient is greatly improved. In the design process, it is necessary to comprehensively consider various factors and then determine the K value.

An example of thermal calculation and heat transfer calculation of single-effect lithium bromide absorption refrigerator

(1) heat calculation

① Known conditions:

1) cooling capacity

2) cooling water inlet temperature℃

3) cooling water inlet temperature℃

4) cooling water inlet temperature℃

5) heating the working steam pressure, relative to the steam temperature℃

② Selection of design parameters

1) absorber outlet cooling water temperature 1 and condenser outlet cooling water temperature 2 In order to save the consumption of cooling water, series connection is adopted. Assuming that the total temperature rise of cooling water =8℃, and taking 1℃ and 2℃, then

2) If the condensation temperature and pressure are℃, then

3) If the evaporation temperature and pressure are both℃, then

4) The lowest temperature of the dilute solution in the absorber is℃, then

5) absorber pressure assumption, and then

6) The concentration of dilute solution is obtained by looking up the map.

7) take the concentration of concentrated solution, and then

8) According to the map, the highest temperature of the concentrated solution in the generator is℃.

9) When the concentrated solution leaves the heat exchanger and the temperature difference at the cold end is℃, then

℃

10) The enthalpy sum of the concentrated solution when it leaves the heat exchanger is found on the diagram.

The temperature of 1 1) dilute solution heat exchanger is obtained by formulas (1) and (12).

According to the sum, find the temperature on the chart.

12) The enthalpy and concentration of the spray solution are obtained from formula (13) and formula (14) respectively, which are taken in the calculation.

Look up the map with sum to get℃

According to the above data, the parameters of each point are determined, and their values are listed in Table 3. Considering the order of magnitude of pressure, the pressure unit in the table is kPa.

③ Calculation of equipment heat load

1) The cooling water flow rate is obtained by formula (15) and formula (16).

2) The thermal load of the generator is obtained by the formula (17).

3) The heat load of the condenser can be known from the formula (18).

4) The heat load of the absorber can be known from the formula (19).

5) The heat load of the solution heat exchanger is obtained by Formula (20).

④ Thermal balance, thermal coefficient and thermal perfection of the device.

1) thermal balance

Absorb heat:

Give off heat:

Very close, indicating that the above calculation is correct.

2) Thermodynamic coefficient is obtained by Formula (22)

3) The average temperature of cooling water and the equilibrium temperature of refrigerant water with perfect thermal power are respectively

By formula (23)

By formula (24)

⑤ Calculation of heating steam consumption and flow of various pumps.

1) The consumption of heating steam is expressed by formula (25).

2) The flow rate of the absorption pump is expressed by formula (26).

In the formula, it can be obtained by looking up the figure and.

3) The flow rate of the generator pump is given by Formula (27).

In the formula, it can be obtained by looking up the figure and.

4) The flow rate of the refrigerant water pump is expressed by formula (28).

5) The flow rate of the cooling water pump is given by Formula (29) and Formula (30).

The two are basically the same, which shows that the total temperature rise distribution of cooling water assumed at the beginning is appropriate and taken.

6) The flow rate of the evaporator pump is given by the formula (3 1), and a= 10. Therefore,

(2) Heat transfer calculation

(1) The generator area is obtained from Formula (34), and then

② The heat transfer area of the condenser is given by Formula (35), and then

③ The heat transfer area of the absorber is given by Formula (36), then

④ The heat transfer area of the evaporator is given by Formula (37), then

⑤ The heat transfer area of the solution heat exchanger is given by formula (38), and then