Chemical Engineering

A chiller is a machine that removes heat from a liquid via a vapor-compression or absorption refrigeration cycle. A vapor-compression water chiller comprises the 4 major components of the vapor-compression refrigeration cycle (compressor, evaporator, condenser, and some form of metering device). These machines can implement a variety of refrigerants. Adsorption chillers use municipal water as the refrigerant and benign silica gel as the desiccant.

Absorption chillers utilize water as the refrigerant and rely on the strong affinity between the water and a lithium bromide solution to achieve a refrigeration effect. Most often, pure water is chilled, but this water may also contain a percentage of glycol and/or corrosion inhibitors; other fluids such as thin oils can be chilled as well. Contents [hide] 1 Use in air conditioning 2 Use in industry 3 Vapor-Compression Chiller Technology 4 How Adsorption Technology Works 5 How Absorption Technology Works 5. 1 Industrial chiller technology Industrial chiller selection 7 Refrigerants 8 See also 9 References 10 External links [edit] Use in air conditioning In air conditioning systems, chilled water is typically distributed to heat exchangers, or coils, in air handling units, or other type of terminal devices which cool the air in its respective space(s), and then the chilled water is re-circulated back to the chiller to be cooled again. These cooling coils transfer sensible heat and latent heat from the air to the chilled water, thus cooling and usually dehumidifying the air stream.

A typical chiller for air conditioning applications is rated between 15 to 1500 tons (180,000 to 18,000,000 BTU/h or 53 to 5,300 kW) in cooling capacity. Chilled water temperatures can range from 35 to 45 degrees Fahrenheit or 1. 5 to 7 degrees Celsius, depending upon application requirements. [1] [2] [edit] Use in industry In industrial application, chilled water or other liquid from the chiller is pumped through process or laboratory equipment. Industrial chillers are used for controlled cooling of products, mechanisms and factory machinery in a wide range of industries.

They are often used in the plastic industry in injection and blow molding, metal working cutting oils, welding equipment, die-casting and machine tooling, chemical processing, pharmaceutical formulation, food and beverage processing, paper and cement processing, vacuum systems, X-ray diffraction, power supplies and power generation stations, analytical equipment, semiconductors, compressed air and gas cooling. They are also used to cool high-heat specialized items such as MRI machines and lasers, and in hospitals, hotels and campuses.

The chillers for industrial applications can be centralized, where each chiller serves multiple cooling needs, or decentralized where each application or machine has its own chiller. Each approach has its advantages. It is also possible to have a combination of both central and decentral chillers, especially if the cooling requirements are the same for some applications or points of use, but not all. Decentral chillers are usually small in size (cooling capacity), usually from 0. 2 tons to 10 tons. Central chillers generally have capacities ranging from ten tons to hundreds or thousands of tons.

Chilled water is used to cool and dehumidify air in mid- to large-size commercial, industrial, and institutional (CII) facilities. Water chillers can be either water cooled, air-cooled, or evaporatively cooled. Water-cooled chillers incorporate the use of cooling towers which improve the chillers’ thermodynamic effectiveness as compared to air-cooled chillers. This is due to heat rejection at or near the air’s wet-bulb temperature rather than the higher, sometimes much higher, dry-bulb temperature.

Evaporatively cooled chillers offer efficiencies better than air cooled, but lower than water cooled. Water cooled chillers are typically intended for indoor installation and operation, and are cooled by a separate condenser water loop and connected to outdoor cooling towers to expel heat to the atmosphere. Air Cooled and Evaporatively Cooled chillers are intended for outdoor installation and operation. Air cooled machines are directly cooled by ambient air being mechanically circulated directly through the machine’s condenser coil to expel heat to the atmosphere.

Evaporatively cooled machines are similar, except they implement a mist of water over the condenser coil to aid in condenser cooling, making the machine more efficient than a traditional air cooled machine. No remote cooling tower is typically required with either of these types of packaged air cooled or evaporatively cooled chillers. Where available, cold water readily available in nearby water bodies might be used directly for cooling, or to replace or supplement cooling towers. The Deep Lake Water Cooling System in Toronto, Canada, is an example.

It dispensed with the need for cooling towers, with a significant cut in carbon emissions and energy consumption. It uses cold lake water to cool the chillers, which in turn are used to cool city buildings via a district cooling system. The return water is used to warm the city’s drinking water supply which is desirable in this cold climate. Whenever a chiller’s heat rejection can be used for a productive purpose, in addition to the cooling function, very high thermal effectivenesses are possible. [edit] Vapor-Compression Chiller Technology

There are basically four different types of compressors used in vapor compression chillers: Reciprocating compression, scroll compression, screw-driven compression, and centrifugal compression are all mechanical machines that can be powered by electric motors, steam, or gas turbines. They produce their cooling effect via the “reverse-Rankine” cycle, also known as ‘vapor-compression’. With evaporative cooling heat rejection, their coefficients-of-performance (COPs) are very high and typically 4. 0 or more. In recent years, application of Variable Speed Drive (VSD) technology has increased efficiencies of vapor compression chillers.

The first VSD was applied to centrifugal compressor chillers in the late 1970s and has become the norm as the cost of energy has increased. Now, VSDs are being applied to rotary screw and scroll technology compressors. [edit] How Adsorption Technology Works Adsorption chillers are driven by hot water. This hot water may come from any number of industrial sources including waste heat from industrial processes, prime heat from solar thermal installations or from the exhaust or water jacket heat of a piston engine or turbine. The principle of adsorption is based on the interaction of gases and solids.

With adsorption chilling, the molecular interaction between the solid and the gas allow the gas to be adsorbed into the solid. The adsorption chamber of the chiller is filled with solid material, silica gel, eliminating the need for moving parts and eliminating the noise associated with those moving parts. The silica gel creates an extremely low humidity condition that causes the water refrigerant to evaporate at a low temperature. As the water evaporates in the evaporator, it cools the chilled water. The use of a benign silica gel desiccant keeps the maintenance costs and operating costs of adsorption chillers low. edit] How Absorption Technology Works Absorption chillers’ thermodynamic cycle are driven by heat source; this heat is usually delivered to the chiller via steam, hot water, or combustion. Compared to electrically powered chillers, they have very low electrical power requirements – very rarely above 15 kW combined consumption for both the solution pump and the refrigerant pump. However, their heat input requirements are large, and their COPs are often 0. 5 (single-effect) to 1. 0 (double-effect). For the same tonnage capacity, they require much larger cooling towers than vapor-compression chillers.

However, absorption chillers, from an energy-efficiency point-of-view, excel where cheap, high grade heat or waste heat is readily available. In extremely sunny climates, solar energy has been used to operate absorption chillers. The single effect absorption cycle uses water as the refrigerant and lithium bromide as the absorbent. It is the strong affinity that these two substances have for one another that makes the cycle work. The entire process occurs in almost a complete vacuum. 1. Solution Pump – A dilute lithium bromide solution is collected in the bottom of the absorber shell.

From here, a hermetic solution pump moves the solution through a shell and tube heat exchanger for preheating. 2. Generator – After exiting the heat exchanger, the dilute solution moves into the upper shell. The solution surrounds a bundle of tubes which carries either steam or hot water. The steam or hot water transfers heat into the pool of dilute lithium bromide solution. The solution boils, sending refrigerant vapor upward into the condenser and leaving behind concentrated lithium bromide. The concentrated lithium bromide solution moves down to the heat exchanger, where it is cooled by the weak solution being pumped up to the generator. . Condenser – The refrigerant vapor migrates through mist eliminators to the condenser tube bundle. The refrigerant vapor condenses on the tubes. The heat is removed by the cooling water which moves through the inside of the tubes. As the refrigerant condenses, it collects in a trough at the bottom of the condenser. 4. Evaporator – The refrigerant liquid moves from the condenser in the upper shell down to the evaporator in the lower shell and is sprayed over the evaporator tube bundle. Due to the extreme vacuum of the lower shell [6 mm Hg (0. kPa) absolute pressure], the refrigerant liquid boils at approximately 39°F (3. 9°C), creating the refrigerant effect. (This vacuum is created by hygroscopic action – the strong affinity lithium bromide has for water – in the Absorber directly below. ) 5. Absorber – As the refrigerant vapor migrates to the absorber from the evaporator, the strong lithium bromide solution from the generator is sprayed over the top of the absorber tube bundle. The strong lithium bromide solution actually pulls the refrigerant vapor into solution, creating the extreme vacuum in the evaporator.

The absorption of the refrigerant vapor into the lithium bromide solution also generates heat which is removed by the cooling water. The now dilute lithium bromide solution collects in the bottom of the lower shell, where it flows down to the solution pump. The chilling cycle is now completed and the process begins once again. [edit] Industrial chiller technology Industrial chillers typically come as complete packaged closed-loop systems, including the chiller unit, condenser, and pump station with recirculating pump, expansion valve, no-flow shutdown, internal cold water tank, and temperature control.

The internal tank helps maintain cold water temperature and prevents temperature spikes from occurring. Closed loop industrial chillers recirculate a clean coolant or clean water with condition addititives at a constant temperature and pressure to increase the stability and reproducibility of water-cooled machines and instruments. The water flows from the chiller to the application’s point of use and back. If the water temperature differentials between inlet and outlet are high, then a large external water tank would be used to store the cold water.

In this case the chilled water is not going directly from the chiller to the application, but goes to the external water tank which acts as a sort of “temperature buffer. ” The cold water tank is much larger than the internal water tank. The cold water goes from the external tank to the application and the return hot water from the application goes back to the external tank, not to the chiller. The less common open loop industrial chillers control the temperature of a liquid in an open tank or sump by constantly recirculating it. The liquid is drawn from the tank, pumped through the chiller and back to the tank.

An adjustable thermostat senses the makeup liquid temperature, cycling the chiller to maintain a constant temperature in the tank. One of the newer developments in industrial water chillers is the use of water cooling instead of air cooling. In this case the condenser does not cool the hot refrigerant with ambient air, but uses water cooled by a cooling tower. This development allows a reduction in energy requirements by more than 15% and also allows a significant reduction in the size of the chiller due to the small surface area of the water based condenser and the absence of fans.

Additionally, the absence of fans allows for significantly reduced noise levels. Most industrial chillers use refrigeration as the media for cooling, but some rely on simpler techniques such as air or water flowing over coils containing the coolant to regulate temperature. Water is the most commonly used coolant within process chillers, although coolant mixtures (mostly water with a coolant additive to enhance heat dissipation) are frequently employed. [edit] Industrial chiller selection

Important specifications to consider when searching for industrial chillers include the total life cycle cost, the power source, chiller IP rating, chiller cooling capacity, evaporator capacity, evaporator material, evaporator type, condenser material, condenser capacity, ambient temperature, motor fan type, noise level, internal piping materials, number of compressors, type of compressor, number of fridge circuits, coolant requirements, fluid discharge temperature, and COP (the ratio between the cooling capacity in RT to the energy consumed by the whole chiller in KW).

For medium to large chillers this should range from 3. 5-7. 0 with higher values meaning higher efficiency. Chiller efficiency is often specified in kilowatts per refrigeration ton (kW/RT). Process pump specifications that are important to consider include the process flow, process pressure, pump material, elastomer and mechanical shaft seal material, motor voltage, motor electrical class, motor IP rating and pump rating. If the cold water temperature is lower than -5°C, then a special pump needs to be used to be able to pump the high concentrations of ethylene glycol.

Other important specifications include the internal water tank size and materials and full load amperage. Control panel features that should be considered when selecting between industrial chillers include the local control panel, remote control panel, fault indicators, temperature indicators, and pressure indicators. Additional features include emergency alarms, hot gas bypass, city water switchover, and casters. [edit] Refrigerants A vapor-compression chiller uses a refrigerant internally as its working fluid.

Many refrigerants options are available; when selecting a chiller, the application cooling temperature requirements and refrigerant’s cooling characteristics need to be matched. Important parameters to consider are the operating temperatures and pressures. There are several environmental factors that concern refrigerants, and also affect the future availability for chiller applications. This is a key consideration in intermittent applications where a large chiller may last for 25 years or more. Ozone depletion potential (ODP) and global warming potential (GWP) of the refrigerant need to be considered.

ODP and GWP data for some of the more common vapor-compression refrigerants: Refrigerant ODP GWP R-134a 0 1300 R-123 0. 012 120 R-22 0. 05 1700 R401a 0. 027 970 R404a 0 3260 R407a 0 ??? R407c 0 1525 R408a 0. 016 3020 R409a 0. 039 1290 R410a 0 1725 R500 0. 7 ??? R502 0. 18 5600 [edit] See also HVAC Cooling tower Evaporative cooling Chemical engineering Mechanical engineering Architectural engineering Building services engineering [edit] References ^ American Society of Heating and Refrigeration Enginneers http://www. ashrae. org/publications/page/158 ^ Hydronika supplies 5 ton chiller units http://hydronika. com

Cleaned the weld chamfers free from rust preventive coating. Welded erection irons (20 Nos) on both sides of the joint & pulled the sections together by draw bolt. Adjusting irons are welded in between the erection irons where shell overlap was found. Checked the alignment of the shell with the Piano wire arrangement. A gap of 3mm maintained in between the shell joint. Strong backs will be welded on one shell after initial alignment. Took polar readings on either side of the joint & both ends of the ShellsPlotted polar diagrams and checked the eccentricity. Tolerances: Eccentricity (radial runout ) must not exceed +/- 1. 0 mm of the inlet & outlet rings +/- 1. 5mm of the erection welds. +/-

1. 4mm of kiln section in the live ring. The axial untruth of live ring must not exceed +/- 1mm After completion of the alignment, weld the strong backs to the other side. Welding of Joint: Clean the joint surface. Preheat the joint area of the shell ( 1m lg ) to 150 deg Welding electrodes are to be preheated Complete the root run on the outer surface of the joint side with 2. 5mm electrode. Next with 4mm electrode & then 5mm & 6 mm electrodes. Gouge & remove the root run weld from the inner surface of the joint, check with Dye penetrant for any cracks and start the root run inside.  Complete the inside welding. Check the joint by Ultrasonic testing. Importance of Pre-heating: Preheating slows the cooling rate in the weld area. This may be necessary to avoid cracking of the weld metal or heat affected zone.

Hydrogen contributes to delayed weld and /or heat affected zone cracking, hence it is important to keep the weld joint free of oil, rust, paint, and moisture as they are sources of hydrogen. Electrodes used ( 1-2 joint) : FLS 9721 : E 7018 (3. 15mm ) Bottom runs FLS 9721 : E 7018 (4. 00mm) FLS 9721 : E 7018 (5mm) FLS 9732 : E 6027 for Cover Run E 7018 E 6027 Yield stress : 410-480360-410 N/sq. mm Tensile strength :510-590 440-490 N/sq. mm Elongation : 28% 27 Impact (Charpy V) +20deg c – approx. 14090 Electrodes used (other joints) : FLS 9727 : E 9018 (3. 15mm ) FLS 9727 : E 9018 (4. 00mm) FLS 9727 : E 9018 (5mm)FLS 9727: E 9018 ( 6mm) E 9018: Yield stress : 490-590 N/sq. mm Tensile strength : 640-740 N/sq. mm Elongation : 25% Impact (Charpy V) +20deg c – approx. 180

In the production of oil from subsurface reservoirs, 65% of the oil initially in place (OIIP), on average, is left in the reservoir after more oil as possible has been recovered by natural depletion and with the aid of water flooding. Residual oil and gas are enhanced oil recovery (EOR) methods.

EOR techniques are classified into thermal (such as steam or hot water injection) techniques and non-thermal techniques (including designer water flooding, gas injection and chemical flooding). The former is primarily intended for heavy oils, while the latter are normally applied in light oil reservoirs.

There are some of the non-thermal enhanced oil recovery methods, such as polymer flooding, alkaline-surfactant-polymer (ASP) and alkaline flooding are much expensive and are also subjected to some operational restrictions, such as temperature (reservoir) and formation permeability.

Gas injection techniques in various forms consisting of hydrocarbon gas injection (including natural gas, enriched natural gas and a liquefied petroleum slug driven by natural gas) and non-hydrocarbon gas injection (such as carbon dioxide, nitrogen and flue gas) are widely used to reduces the residual oil saturation.

In gas injection, a compressed gas such as carbon dioxide (CO2), natural gas (consisting primarily of methane, CH4), nitrogen (N2), or flue gases are injected into the reservoir to displace oil toward the production wells. The injected gas either partially dissolves in the oil (immiscible gas flooding) or mixes completely with it (miscible flooding), leading mainly to swelling of the oil, viscosity reduction in the oil phase and also for miscible flooding, lowering of the interfacial tension (IFT) between the displacing phase and oil .

CO2 injection is preferred because it applies for two different purposes; improving oil recovery and CO2 sequestration for diminish the greenhouse gases emissions. Several problems such as corrosion in the production wells or injection and surface facilities as well  , CO2 separation from the saleable hydrocarbons, large requirement of CO2 per increase in barrel and asphaltene precipitation which causes formation damage and wettability alteration have been reported for CO2 injection process.

Injection of N2 or nitrogen-contaminated lean hydrocarbon gases are appropriate EOR processes for deep reservoirs, high pressure reservoirs, with light or volatile oil that are rich in light and also intermediate hydrocarbon components (C2–C5) due to their miscible displacement potential. Low cost, abundance and availability of nitrogen are the most reported advantages for nitrogen injection.

Nitrogen is produced by cryogenic processes from air for a long period of time.CO2 (carbon dioxide) flooding enhances oil recovery by the following main mechanisms: (1) oil swelling, (2) reduction of crude oil viscosity, and (3) reduction of interfacial tension (IFT), the latter pertains to miscible flooding .

The mechanism of swelling of oil by carbon dioxide injection which makes the volume of oil increase would help discontinuous oil droplets trapped in a porous medium to merge with the flowing oil phase. Reduction in the viscosity is another major mechanism which is significant at even moderate pressures. The amount of solution gas or oil ratio in case of nitrogen injection is lower than that of CO2.

The swelling factors of N2 were also lower than those of CO2 due to nitrogen lower solubility in the oil. If the pressure is low (lower than 3 MPa), solubility of nitrogen and flue gas is negligible. The viscosity reduction due to N2 injection is much lower than that of carbon dioxide injection. Addition of N2 to the injection gas implies that some mechanisms other than swelling and viscosity reduction are important.

One possibility is the buildup of free gas saturation with the N2 containing injectants that may decreases the relative permeability to water, thereby improving the mobility ratio. Moreover, nitrogen has a higher molar volume than CO2 which tells that one mole of nitrogen displaces a higher volume of gas than that of CO2. Therefore, N2 is more favorable in terms of displacement volume. So that our focus in this study is on N2.Literature review on N2 miscibility

Immiscible gas injection can potentially recover a large amount fraction of the remaining oil after primary depletion or water flooding (WF). However, such potential has hardly ever been realized because of the low vertical efficiency and areal sweep efficiency. Nitrogen injection process is also performed either by miscible or immiscible, depending on the injection pressure of N2, reservoir temperature and reservoir oil composition. Miscibility is theoretically defined as the conditions at which there is no interface between the reservoir oil and displacing phase .

In other words, it can be say that two phases are miscible when a single phase fluid is produced after intermingling of two fluids with each other at any ratio. The lowest operating pressure, at reservoir temperature, at which miscibility is achieved between reservoir fluid and injection gas is termed as the minimum miscibility pressure (MMP) . There has been a few correlations in the literature for N2 MMP estimation producing different average absolute error values.

A study done by Fathinasab, Ayatollahi and Hemmati-Sarapardeh  had resulted in a correlation for MMP which will be used for pure N2, nitrogen mixtures and lean gases. The developed correlation yields the least error and is a function of average critical temperature of the injection gas, reservoir temperature, C7 + fraction molecular weight of crude oil, volatile components (mole fraction) and intermediate components (mole fraction) of crude oil.

Since N2 is not as good a solvent for oils as carbon dioxide (CO2), or even methane (CH4), the pressure required for nitrogen to become miscible with any oil should be greater than that for methane which, in turn, is higher than CO2 . This especially makes nitrogen attractive for highly undersaturated reservoirs at immiscible conditions.

Literature review on challenges in gas flooding and a solution

The major technical challenge of immiscible gas injection is to maintain proper sweep efficiency of the injected gas, improve gas utilization and delay its breakthrough. These result from a combination of gravity override and gas channeling through high permeability streaks in the formation. Gas segregation, channeling and fingering through high permeability streaks are inherent in any gas injection; they are due to the excessively higher mobility and far lower density of gas (displacing phase) compared to oil or water (displaced phase).

Unfavorable mobility ratios lead to even more severe channeling in heterogeneous reservoirs and heavier oil reservoirs. Consequently, the drive fluid does not contact a large part of the reservoir and the volumetric sweep efficiency of the reservoir remains poor .

Furthermore, a displacement is adversely affected by capillary end effects, arising from the discontinuity of capillarity in the wetting phase at the outlet end of the core, that, for the gas/oil system, cannot be overcome by high gas throughput rates. WAG injection is implemented to improve mobility ratio and sweep efficiency.