Process cooling is accomplished by circulating a fluid, at a temperature lower than the process, through heat exchangers. The process determines the fluid circulation method required. In cases where a liquid is being circulated, a mechanical pump is employed to provide the transportation energy to move the liquid. Other processes are based on a vapor or gaseous mixture, which require either a compressor or fan to provide the fluid transportation energy.
Process cooling requirements and refrigeration systems can be categorized by the heat transfer mechanism employed: sensible heat transfer, latent heat transfer, or a combination of both. The mode of heat transfer depends on the nature of the process requirements. In all cases, the primary working fluid - the refrigerant - undergoes a phase change from liquid to vapor. The secondary fluid is the process fluid itself or an intermediate fluid.
Process refrigeration systems are provided by manufacturers in package configurations that vary depending upon the physical limitations imposed by shipment constraints or job-site allowances. A complete skid-mounted package includes all required components on a single structural skid and requires only utility connections such as water and electricity. Higher capacity, more complex systems usually require a modularized design due to the size of the skids or components. These modules are fixed in place at the job site. Then, the piping between modules is field erected and the utility connections are made.
Self-contained skids or modularized versions are provided with a full complement of the required safety, operating and flow control devices. Specialized coatings may be used to provide base material protection from corrosive environments, and insulation systems may be factory installed to prevent heat gain or protect personnel.
These systems are designed based upon the process operating conditions, which will dictate the selection of the primary heat absorption and rejection components, known as the evaporators and condensers. Process operating conditions cover a range of refrigeration system design requirements such as process heat loads and operating pressures, hazardous or nonhazardous area classification, ambient conditions, and available utilities such as water, electric and pneumatic air lines.
Defining RefrigerationRefrigeration is a means of transferring heat from one area to another area where the heat release is unobjectionable. Because energy or heat only flows downhill, two heat sink temperatures are necessary for heat transfer to occur. The first heat sink is the refrigeration system evaporator. It is maintained at a temperature below the process temperature requirement. Heat flows from the higher process temperature to the evaporator. The second heat sink is the ambient condition into which heat energy is rejected. A process's ambient condition determines the type of heat rejection device suitable for the project. The heat rejection device is the condenser. For refrigeration systems, it operates at a temperature level higher than the ambient condition. It releases the heat transferred from cooling of the process to the high temperature heat sink (the ambient condition), which is at a temperature lower than the condenser.
Each heat sink will require a heat transfer step. For each step of heat transfer added to the process cooling cycle, the refrigeration system will become less efficient. Thus, the process cooling application and the overall system design will determine the number of heat transfer steps and refrigeration system efficiency.
Types of Refrigeration SystemsRefrigeration system types can be allocated into fluid chillers and condensing systems categories. Fluid chiller systems provide fluid cooling through a heat ex-changer, where liquid or vapor is circulated. The process fluid may be recirculated in a closed loop chilled water system through which the fluid is pumped through the piping. In some liquid applications, brine is utilized when the process temperature requirements are below the fluid's freezing point. The brine solution and level of concentration depress the freezing point of the fluid below the process-operating required temperature to prevent potential component damage (figures 1 and 1a).
Sensible heat transfer is performed when no change of state or phase of the circulated fluid occurs. A mechanical pump circulates the liquid through a piping network designed to provide the required flow rate to the desired location. This mode of heat transfer usually is employed for applications involving large, complex building structures or vast differences in locations between the process refrigeration system and the heat exchangers. The fluid's specific heat is based upon the selected fluid, fluid type and concentration, and its operating temperature. The required process temperature and a balance among the amount of heat exchange surface, supplied mass flow and the fluid's specific heat determine temperature difference. Energy must undergo a number of heat transfer steps from the process fluid to the refrigerant. Modularized refrigeration systems provide chilled liquid to a location within the plant where the process is installed. Fluid chiller systems also provide fluid cooling through a heat exchanger, where air or a gas mixture is cooled and liquid is condensed from the vapor stream.
Condensing systems are those where a remote evaporator is cooled by a phase change in the working fluid. For this discussion, assume the working fluid is a refrigerant. Latent heat transfer is accomplished when the circulated fluid undergoes a change in state. Phase changes can operate in two separate modes: condensation, which is a heat release mode, or evaporation, which is a heat absorption mode. Both modes of phase change are dependent upon the process of transferring heat. With this application, the circulation of the working fluid is by the main refrigeration compressor and is distributed through a piping network to the various evaporators or condensers.
Condensing systems typically are used where the process heat exchangers are located within the processing plant and the refrigerant is piped directly to the process heat exchangers. Liquid refrigerant is supplied to various ex-changers or distillation columns, where it is expanded and boils. Boiling occurs due to the absorption of heat transferred by the process through the heat exchanger surface. A condensing system generally is more energy efficient than a fluid chiller due to the elimination of one heat transfer step (figures 2 and 2a).
Packaged process cooling systems are custom engineered to meet exacting process conditions and can be used in any process industry where temperature requirements are lower than those easily obtained by other means. Savings of both time and money can be realized by using packaged process cooling systems.
End-users have specific and intensive requirements for specification compliance, and they utilize these systems to provide dependable and cost-effective solutions for their process cooling requirements. PCE