Experimental conditions from Ref. .
Modifying the steam plant condenser by incorporating a vapor compression refrigeration system, which is utilized to chill the water used for cooling the condenser air cooler, is proposed in this work. Hence a considerable drop in air temperature is obtained, and more steam associated with air is condensed resulting in reducing both steam lost and vacuum pump power. A thermodynamic analysis is developed for working out the performance of the proposed hybrid system. The results obtained using this analysis showed that for a condensate subcooling of 3°C the total work saving due to this modification has maximums of 38.5, 33.9 and 28.9% and occur at temperature reduction below the condensate temperature of 12, 10, and 8°C when the temperature of steam admitted to the condenser is 20, 30, and 40°C, respectively. The savings in steam lost through the vacuum pump in these cases amount to 87.7, 84, and 79.2%, respectively. It is recommended to select the decrease in condenser air temperature in the range of 4°C higher than those decreases, at which the minimum total work comes about, as the saving in steam lost becomes fairly greater (5–7%), while the maximum increase in minimum total work is 3%.
- steam plant condenser
- vapor compression refrigeration system
- condenser air cooler
- vacuum pump
- power saving
The primary purpose of the steam condenser in a steam plant is to convert the turbine exhaust steam into water for reuse in the boiler and to create and maintain vacuum at the turbine exhaust. This is achieved by cold water circulation through tube bundles and condensing the steam on their outer surfaces. Referable to the large reduction in the specific volume on changing from steam to condensate, a vacuum is formed within the condenser. Getting to this vacuum continuously as minimum as possible is very important for the steam plant to attain the highest possible power from the turbine. A rise in the condenser pressure can be produced due to the existence of air, ammonia, and other noncondensable gases . This rise in pressure in the condenser results in lowering the useful turbine power and in turn the plant efficiency degenerates [2, 3, 4, 5].
The presence of air lowers the partial pressure of steam and therefore brings down the saturation temperature of steam, which leads to increasing evaporation enthalpy (latent heat), and therefore, more cooling water will be required in the condenser. In addition, this gives rise to increasing condensate subcooling, which is undesirable because it means that the excess heat removed for this purpose serves no useful process and entails providing additional boiler firing . Thus, continuous removal of the air and noncondensable gases from the condenser is very critical for maintaining the plant efficiency possibly high. Beside this task, the air extraction system prevents air blanketing of condenser tubing that could dramatically reduce the heat transfer and stop the condensing process. Also, it reduces the condensate-dissolved oxygen levels that could lead to corrosion of boiler tubing.
The air and noncondensables are extracted from the bottom of the condenser, where the temperature is the lowest, by using vacuum pump. The air and noncondensables sucked by the vacuum pump contain a relatively big amount of noncondensed steam; each kilogram of air can contain more than 2 kg of water vapor . This results in increasing the parasitic work of the pump. In condenser design, it is considered reducing the subcooling of the condensate possibly to the lowest degree to minimize heat removal. For reducing the amount of steam mixed with the air sucked by the air pump, the steam condenser has a portion of tubes near the air pump suction screened off and the condenser tubes in this portion contain the coldest water, and the number of cooling tubes is increased. Hence, the temperature of the air drops below that of the outlet condensate, and more steam associated with the air is condensed so that the mass flow rate of the air and steam mixture drawn by the air pump is reduced and, in turn the power of the dry pump is lowered.
The air extraction system (vacuum pump) can be centrifugal compressor, steam jet ejector air ejector, or water ring pump [8, 9]. A hybrid system of these components is preferred as they offer several advantages toward minimizing venting equipment size and save parasitic consumption . The power required for operating these equipment sorts is dependent on both the rates of air and the noncondensables and noncondensed steam sucked by the vacuum pump. The temperature decrease of the air in the condenser air cooler is limited when the normal cooling water is used. For obtaining considerable drop in air temperature it is proposed in this work to modify the condenser by incorporating a refrigeration system into the condenser. This system is utilized to chill the cooling water, which is used for cooling the air in the condenser rather than the normal cooling water. Of course the use of the refrigerant directly to cool the air exploiting the air cooler as an evaporator for the refrigeration system and circulating the cold liquid refrigerant of this system through the tubes of the air cooler will be more efficient than using the chilled water. However, this configuration calls for redesigning the air cooler to be adapted for receiving and evaporating the refrigerant, which will be expensive for already existing power plants. The latest configuration can be taken into consideration for the design of the new plants.
The concept of using dry ammonia for cooling steam plant condenser is described in Ref. . This concept states that the condenser cooling water is replaced by liquid ammonia, which evaporates as it acquires heat from the condensing steam. The heat of condensation absorbed by the ammonia is rejected in an air cooled condenser of a refrigeration machine into the surrounding atmosphere. This concept was tested and well documented [11, 12, 13, 14, 15, 16, 17], with the participation of several major equipment vendors (Baltimore Air Coil, the Trane Company, Curtiss-Wright, CB&I, and Union Carbide). In addition, the concept of using vapor compression refrigeration system (VCRS) combined with steam plant condenser was studied in detail in [14, 15, 16, 17, 18, 19]. The results of these studies showed significant rise in net power as well as efficiency of the steam plant. However, for realizing this combined system, a relatively huge VCRS is required, which leads to material increase of the combined system cost. In the current study, a relatively small VCRS is required as it serves to cool only the air contained in the air cooler of steam plant condenser. This leads to lessening the temperature of the air and water vapor mixture in the air cooler, resulting in condensing some of the water vapor. Hence, the mass flow rate of the air and water vapor mixture extracted from the steam condenser is decreased and consequently the power consumed by the vacuum pump lessens. Meanwhile, water vapor leaving the steam condenser and lost to the surroundings is reduced. In general, this contributes to contracting both the power consumed in the auxiliary systems of the steam plant and the steam consumption.
2. Conception of the modified steam plant condenser
The steam plant condenser is typically a shell and tubes heat exchanger. Figure 1 depicts a section through a steam plant condenser. The steam leaving the plant turbine enters the condenser at its top with temperature
Figure 2 illustrates a configuration for the modified steam plant condenser that enables the use of chilled water to lower the temperature of the air cooler section. The cooling water (a) coming from a water body (river, sea, …) or a cooling tower is fed into the header (e) where it is distributed to the most of the cooling water tubes of the condenser. It helps to cool the steam exhausting out of the plant turbine. This cooling water is called here normal cooling water. While being heated up, it exits the cooling tubes into the header (f). The warmer normal cooling water (b) is extracted from the header (f) and pumped back to the water body or the cooling tower. The chilled water (c) is fed to the header (g) and it flows through the cooling tubes of the air cooling part of the steam condenser. This chilled water acts as a coolant for air along with some water vapor collected in the air cooler. This leads to cooling the mixture of the air and water vapor to a temperature lower than that of the condensate. The temperature of the chilled water is raised as it flows through the cooling tubes due to heat transfer from the mixture of air and steam in the air cooler. As a result, some of the steam contained in the mixture is condensed and flows down the condenser to the hot well. The warm chilled water goes into the header (h), from which it is transmitted backward to a chiller. Finally, the rest of the air and water vapor mixture in the air cooler is sucked by a vacuum pump and expelled into the atmosphere. The vacuum pump is not shown in Figure 2.
3. Description of the studied combined system
In the current study, a vapor compression refrigeration system (VCRS) is used to chill the cooling water used for cooling the air cooler of the above described steam condenser. Hence, Figure 3 shows a schematic of the studied combined system. The refrigeration cycle is made up of a two-stage compression system (c) and (d) (these are also designated by I and II, respectively); a refrigerant condenser (e); two throttling valves (f) and (h); a flash tank (g) for intercooling and flash gas removal; a liquid-line/suction-line heat exchanger (LLSL-HE) (b); and an evaporator (a), which acts at the same time as a heat exchanger for chilling the cooling water used in the air cooler. In this system, the low-pressure refrigerant leaving the evaporator (a) is heated in the liquid suction heat exchanger (b) as it absorbs heat from the higher-temperature liquid refrigerant coming from the flash chamber (g). It is drawn by the first-stage compressor (c) where its pressure is raised to the intermediate pressure of the flash chamber. It is sent to the flash chamber (g) in which it is mixed with the refrigerant coming out of the throttle valve (f) and it gets cooled. The refrigerant vapor is separated from the liquid refrigerant and it is drawn by the second-stage compressor (d) and its pressure is elevated to the condenser pressure. The high-pressure refrigerant leaving the compressor (d) flows through the refrigerant condenser (e) where it is condensed and it streams further to the throttle valve (f) where its pressure is reduced to the intermediate pressure of the flash chamber, and it is fed into the flash chamber. The liquid refrigerant leaving the flash chamber is fed to the liquid suction HE (b). Here it is cooled as it gives a portion of its sensible heat to the refrigerant vapor departing the evaporator (a). The cooled liquid refrigerant is reduced in pressure to the evaporator pressure on running through the throttle valve (h). The refrigerant is then run to the evaporator/water chiller to cool down the warm water coming from the air cooler. The low-pressure vapor refrigerant leaving the evaporator/water chiller (a), completes the refrigeration cycle. This refrigeration cycle has been selected among the available refrigeration cycles due to its simplicity and relatively high efficiency.
4. Thermodynamic analysis of the studied combined system
For the following analysis of steam and air flow through the plant condenser, it is assumed that both fluids are ideal gases. Since the steam and air flow inside the condenser are at relatively very low pressure, they can be considered behaving to acceptable accuracy as ideal gases. Referring to Figures 1 and 2, the steam exhausting the plant turbine (it is usually slightly wet) enters the condenser with temperature
The volume flow rate at inlet to the condenser, by Dalton’s law, is equal to the volume flow rate of the associated air  and is obtained from the following equation:
The partial pressure
The total pressure
The condensate temperature
The volume flow rate of the air to be dealt by the vacuum pump at the exit of the steam condenser can be determined by:
Hence, the mass flow rate of steam associated with the air is assessed by:
In the case of screening the steam condenser and cooling the air, the temperature
Regarding the VCRS, the
For obtaining the value of the mass ratio ζ of the chilled water flow rate through the air cooling segment and steam flow rate crossing the turbine, Figure 5 shows the mass flow rates of air mixture and chilled water, and their temperatures through this segment. An energy balance for the air cooler leads to the following equation:
It is to be noticed here that the sensible heat of the condensed steam in the air cooler was neglected as it is relatively very small, and is equal to .
Solving Eq. (13) to get and multiplying both sides of the resulted equation by , the ratio () is obtained as:
For determining the mass ratio of the refrigerant and cooling water, a heat balance is performed for the evaporator/water HE (a), which yields to:
From which it follows that:
Multiplying both sides of Eq. (16) by the mass ratio , the mass ratio
It follows from Eq. (18) that:
Multiplying both sides of Eq. (19) by , it follows that the mass ratio
The characteristic parameters describing the performance of the refrigeration cycle are given as:
The hybrid system proposed in the current work can lead to two benefits, the first benefit is a decrease in the mass flow rate of steam lost in venting the air from the SPC, and the second one is a reduction in mass flow rate of air and steam mixture drawn by the vacuum pump and hence the pump power is lowered. However, a new energy consumption comes out, which is the total work (
When the steam condenser is fitted with air cooler, the air extracting compressor work
5. Results and discussion
The thermodynamic analysis developed in Section 4 for predicting the condensation rate in the steam plant condenser due to air cooling is first validated with the experimental data of reference . In this work, experiments were conducted in a 2-m-long square cross-sectional channel (0.34 m × 0.34 m) to study the heat and mass transfer in the condensation of water vapor from humid air. The air flowing inside the channel was cooled by cold water flowing outside and adjoining only one side (0.34 m × 2 m) of the channel. Experimental data were obtained from five tests at various operating conditions as shown in Table 1. The thermodynamic analysis in Section 4 was slightly modified to be adapted for using the data given in Table 1 for computing the condensation rate. For solving the equations in this analysis and finding out the rate of condensation, the commercial computer package EES  was used. The thermal properties of the humid air at different conditions were found using the built-in functions available in the EES package. The condensation rate computed from the present model is displayed in Figure 6, which shows satisfactory agreement with the experimental data of Ref.  since the maximal discrepancy does not exceed 10%.
|Test||Inlet humid air temperature ||Velocity of humid air ||Inlet relative humidity of humid air ||Average cooling flux |
The thermodynamic analysis developed in Section 4 for performance prediction of the combined system proposed in this work necessitates knowing some basic design and operational data, which is listed in Table 2. It is to be noticed here that the values of isentropic and mechanical efficiencies of the compressors involved in this study have been selected close to the practical values of compressors in use in industry . The results presented hereafter are based on these data. All parameter values given in Table 2 will be kept unchanged except for the case where the effect of a specific parameter is to be examined; it is handled as a variable.
|Mass ratio (||0.0003|
|Compressor isentropic efficiency (||0.85|
|Compressor mechanical efficiency (||0.75|
|Dryness fraction of steam entering the SPC (||0.9|
|Effectiveness of the liquid suction HE (||0.8|
|Temperature difference (||0|
|Temperature difference (||4|
|Temperature difference (||4|
|Temperature difference (||4|
|Temperature difference Δ||3|
|Subcooling of the refrigerant condenser (Δ||3|
The refrigerant of the refrigeration system is selected to be ammonia. The physical properties of air, water, steam, and ammonia needed for computation are predicted using the built-in functions of the commercial computing package EES , which is used for solving the equations of the analysis of Section 4.
In Figure 7, the mass ratio
It is worth noting here that in Figures 8
Figure 9 illustrates the saving percentage (
It follows from Figures 8 and 9 that the amount of steam to be condensed in the air cooler is relatively high at small values of Δ
In Figure 12, the coefficient of performance COP of the refrigeration system is plotted versus the temperature difference Δ
The specific works
The saving percentage (
It is to be mentioned here that the parameters
The current work is concerned with the use of vapor compression refrigeration system (VCRS) for chilling cooling water used with the air cooler of the steam plant condenser (SPC). A thermodynamic analysis is developed for working out the performance of the hybrid system of VCRS and SPC. The results obtained using this analysis showed that subcooling of the SPC condensate can cause considerable reduction in steam rate associated with the air induced by the vacuum pump. However, this is possibly avoided as it represents heat loss in the condensate heat content, which should be compensated in the plant boiler. In addition, the results of this work led to drawing the following conclusions for condensate subcooling of 3°C, which represents a reasonable and practical subcooling of the condenser condensate.
Temperature reductions of the condenser air of 5, 10, and 15°C below the condensate temperature result in reducing steam rate lost in venting air from the condenser relative to the loss when using no air cooler, by around 69, 85, and 90%, respectively.
The total work saving when using chilled water for cooling the air in the condenser air cooler from that in case of no air cooling is applied, has maximums of 38.5, 33.9, and 28.9% and occurs at temperature decrease below the condensate temperature of 12, 10, and 8°C when the temperature of steam admitted to the condenser is 20, 30, and 40°C, respectively. In these cases, the savings in steam lost in venting process amount to 87.7, 84, and 79.2%, respectively.
Selecting the reduction in condenser air temperature in the range of 4°C higher than that temperature reduction, at which the minimum total work occurs, is very advantageous where the saving in steam lost becomes fairly greater while the saving in total work is slightly lower than the minimum total works; in the range of 5–7 and maximally 3%, respectively.
coefficient of performance of the refrigeration system
specific heat capacity at constant pressure (kJ/kg K)
specific heat capacity of water (kJ/kg K)
specific enthalpy (kJ/kg)
specific latent heat (kJ/kg)
mass flow rate (kg/s)
relative heat transfer rate (kW/kg)
gas constant (kJ/kg K)
specific volume (m3/kg)
air velocity (m/s)
volume flow rate (m3/s)
dryness fraction (−)
specific work (kJ/kg)
mass ratio (−)
mass ratio (−)
mass ratio (−)
temperature difference of steam at condenser inlet and exit (K/°C)
temperature difference of chilled water at air cooler inlet and saturated refrigerant in VCRS evaporator (K/°C)
subcooling in the refrigerant condenser
temperature difference of condensate in hot well and air entering the vacuum pump (K/°C)
heat exchanger effectiveness (−)
mass ratio (−)
isentropic efficiency of the compressor (−)
mechanical efficiency of the compressor (−)
mass ratios,, and , respectively (−)
relative humidity (−)
with air cooler
air cooler exit for chilled water
air cooler inlet for chilled water
steam condenser, cooling
steam condenser outlet
steam condenser inlet
air and steam mixture, mechanical
steam turbine, total pressure, total work
vacuum pump inlet
without air cooler
state numbers of the refrigerant of the refrigeration cycle
refrigerant compressor no. I and II, respectively
liquid-line/ suction-line heat exchanger
steam plant condenser
vapor compression refrigeration system