Condensate water recovery system
Abstract: on the basis of theintroduction about the recovery mode of condensed water recovery system and therecovery equipments that are commonly used at home and abroad, this paper isfocused on the discussion on how to select recovery system and equipment, andthe heat economy of condensed water recovery system by examples in order todemonstrate that remarkable energy-saving achievements can be made by the recoveryof condensed water.
Keyword: Condensed Water RecoverySystem, Heat Economy, Recovery mode
As a heat carrier which is widely usedin the industrial circles such as power generation, petroleum, chemical,papermaking, light textile, brewing, rubber and ceramics, the steam can becomesaturated condensed water under almost the same temperature and pressure afterreleasing latent heat of vaporization in steam equipment; the working pressureof steam is higher than its atmospheric pressure, that’s why the heat ofcondensed water can be 20%-30% of steam; the condensed water will have moreheat and higher proportion in total heat of steam along with the increase ofpressure and temperature. It can be inferred that there will be greatpotentials for energy saving if the heat of recovered condensed water can befully recovered and utilized.
For purpose of saving energy, protectingenvironment and reducing production cost by waste-heat utilization, manyenterprises have paid attention to condensed water from production because ofits increasing economic benefits, social benefits and the enterprises’ problemscaused by energy deficiency. Technological upgrading projects on condensedwater recovery and utilization are being continuously launched, but the issueon how to optimize social benefits by effective treatment and utilization ofcondensed water needs further researching.
2 Characteristics of condensed waterrecovery system
Condensed water recovery systemgenerally includes open type and closed type.
2.1 Open type recovery system
In open type recovery system, thecondensed water recovered is sent to water supply tank of boiler and one end ofrecovery pipeline, generally the water collecting tank of condensed water, isopen to atmosphere during recovery and utilization of condensed water. A pumpcan be used to feed condensed water which has insufficient self-pressure toreach the utilization place. The system has merits including simple design,easy operation and small investment and demerits including poor economicbenefits and easy corrosion of equipment, which is caused by high concentrationof dissolved oxygen in condensed water which directly contacts atmosphere. Thissystem is applicable to small steam supply system and the system which has lesscondensed water and secondary steam. Efforts shall be made to try to reducesmoking when using this system to reduce thermal pollution, working medium andenergy losses.
2.2 Closed type recovery system
In closed type recovery system, thewater collecting tank and all pipelines in closed system have constantlypositive pressure. The condensed water will have most of its heat directlyrecovered into boiler through certain equipment and only suffer fromtemperature loss during pipe network and can reduce the treatment cost of waterrecovered in boiler based on high water quality guaranteed by closed system. Ithas such advantages as good economic benefits and long service life but alsodisadvantages including large investment and difficult operation.
The condensed water recovery systemshall be specifically classified according to status of project, such as sitecondition, state parameters and recovery purpose and the open and closed typeare merely the general classifications of condensed water recovery system.
3 Selection of recovery mode andequipment
In different condensed waterreconstruction projects, recovery mode and equipment are critical factorswhether the investment objectives can be reached. Firstly, amount of condensedwater and its drainage from condensed water recovery system obtained must beaccurate to select condensed water recovery system correctly; otherwise, thediameter of condensed water pipes can be improper. Secondly, as the key factorsin selecting condensed water recovery system including system and equipmenttype, pipeline arrangement method and whether secondary steam shall be used andall heat of condensed water shall be recovered, the pressure and temperature ofcondensed water obtained must be accurate. Thirdly, selection of drain valve isalso important in condensed water recovery system since the valve of impropertype may affect pressure and temperature of condensed water utilized and alsoaffect the steam leakage of recovery system.
The possibility for condensed waterrecovery system to effectively utilize different resources in condensed wateris greatly strengthened with the constant improvement of condensed watertechnology and equipment and development of new and high-performance recoveryequipment. For recovery system which generally uses drain valve, watercollecting tank and ordinary water pump as recovery equipment, its problemssuch as water hammer in pipeline caused by coexistence of steam and water,steam leakage caused by improper drain valve, cavitation erosion occurredduring operation of ordinary pump and low utilization of condensed water aregradually solved along with research and development of recovery equipment. Forexample, the pipeline is provided with flash tank of condensed water to fullyutilize the steam contained in water by generating secondary steam throughflash evaporation of condensed water and recovering such steam in order tofully utilize the energy and solve the water hammer in pipeline. What’s more, thehigh-temperatureand saturated condensed water airtight recovery device, which isdesigned based on principle of boosting pressure through ejection and advancedtechnology abroad, can solve the cavitation erosion occurred when centrifugalpump is pumping high-temperature and saturated condensed water and wheninjection pump injects and boosts pressure, fully utilize the thermal energy ofcondensed water in closed type recovery system, recover condensed water to themaximum, save fuel and softened water and make it possible to improve theeconomic efficiency of condensed water recovery system. Some other equipmentsuch as JCRS heat pump condensed water recovery device, which is based on “heatpump” pumping, flash technology and utilizes steam-jet heat pump, can boost thepressure of flash steam from condensed water, recover both steam and water andmake the available steam more than air supply of boiler. It can have remarkableenergy-saving effects by reducing temperature of condensed water when flash steamis suck out and reusing it through anti-cavitation pump; besides, the steamrecovery compressor, which is provided with pressurizing device likepressurizing ring, can directly feed steam and high-temperature condensed waterinto boiler in a high-temperature manner, thus having higher heat recoveryefficiency.
Recovery efficiency of condensed waterrecovery device is greatly increased by improved condensed water recoverydevices, which shall be selected by considering not only the site conditions,but also the steam conditions such as steam pressure and temperature, recoverymode of flash steam and drain valve type.
In addition to the higher recoveryefficiency which contributes to an appropriate system, the heat economy shallbe also a factor considered when recovery purpose of system is satisfied, thatis to say, both waste-heat utilization efficiency and initial investment shallbe fully considered and a optimized plan of engineering project shall havereasonable ratio between input and recovery which is obtained through economicand technical comparison. The closed type recovery system is always preferredin projects for its high efficiency and less pollution to environment.
4 Analysis on heat economy of condensedwater recovery project
In closed type condensed water recoverysystem, the main investment contents include replacement or addition of drainvalve of steam equipment; recovery equipment, such as pump, water collectingtank, heat exchanger, flash tank and high-performance recovery device, as wellas heat preservation and pipe network materials, technical service andengineering construction costs, all of which constitute the entire engineeringinvestment; the investment shall be determined according to site conditions andfeasibility analysis of project. The analysis points for economic benefits ofrecovery project are as follows:
(1) The closed type recovery systemoperates in closed space, which can generate benefits by increasingbackpressure and reducing steam leakage;
(2) Value from recovery of condensedwater and saving of softened water
(3) Benefits can be produced by reducedfuels, which are saved by increasing water inlet temperature of boiler throughimproving the recovery temperature of condensed water.
The condensed water recovery also hasthe social benefits such as reduction of evaporating, emitting, dripping orleaking of steam and condensed water and pollution caused by wastewaterdischarge. Obviously, the benefit and investment of condensed water recoverysystem have certain relationships. The heat economy problem to be considered incondensed water recovery system is how to find the optimized point betweenproject benefit and investment. Investment recovery period is generally adoptedin the engineering technology to check the reasonability and feasibility ofproject investment. A recovery system may have outstanding social and economicbenefits after the problems of energy saving and economy of condensed waterrecovery project are fully considered.
One printing and dyeing mill, forexample, implements processes such as drying and boiling by using latent heatof vaporization released from steam and directly discharges the steam intoditch without recovery and utilization. There will be high production costsince a lot of fuels will be consumed when the softened boiler feed waterenters deaerator with quite low temperature, consumes a lot of high-temperaturesteam and increases the steam production of boiler. Closed type condensed waterrecovery plan is adopted by this mill to reduce production cost, save water andenergy, the steam from dye vat becomes condensed water with certain temperatureand pressure through drain valve, then it enters water collecting tank forflash evaporation and then enters deaerator of boiler to heat the feed water;the condensed water, which comes from water collecting tank and is sent todeaerator through anti-cavitation pump, will be recovered together with itsheat. Bypass is provided to press the condensed water into softened water tankbefore entering boiler. This printing and dyeing mill enjoys enormous economicbenefits after launching this project. After the transformation, this mill hasa reduction of steam leakage of 864t/a from steam equipment, or increase of RMB61,000 as benefit when calculating by taking one year and RMB 70/t of steam asunit separately; the benefit from softened water saved is RMB 58,000/a and thebenefit from fuels saved by improving boiler inlet temperature is RMB302,000/a. The total benefits per year is RMB 420,000/a and the investmentrecovery is realized in less than ten months after the project is launchedaccording to economic and technical comparison since the total cost includingequipment investment is RMB 350,000/year, but its social benefits areinvaluable since the environmental pollution problems caused by evaporating,emitting and leaking are radically removed.
As an important energy-saving measure,the condensed water recovery will be highlighted more seriously since theenergy saving becomes increasingly important along with the decrease of worldenergy. The energy saving benefits of condensed water recovery will be moreimportant along with the wide application of steam energy and constantimprovement of condensed water recovery technology and R&D of condensedwater recovery equipment.