Fluid Heat Transfer Systems Information
Fluid heat transfer systems are packaged units which circulate thermal fluid through process equipment and provide an indirect heating source to maintain an even, consistent temperature profile. They consist primarily of tanks, pipes, pumps, and heating and/or cooling sources. They operate over a wide range of process temperatures at low pressures.
Heating or Cooling
The most fundamental distinguisher of different types of fluid heat transfer systems is whether they are used for maintaining cold or hot fluids. Some systems used exclusively for heating can be further classified based on their primary heating source. These sources include electricity, natural gas, propane, steam, fuel oil, and solar energy. Systems designed for heating and cooling (or just cooling) typically use either air or water for removing heat.
Important specifications include operating temperature, media flow rate, discharge pressure, heater capacity and expansion tank capacity.
Discharge pressure is the required pressure of the fluid in the system. This rating defines the pressure of the pump for pumping fluid through the system. It is typically measured in pounds per square inch (psi).
Heater capacity is the ability of a device to add heat to the process fluid. It is typically measured in kilowatts (kW) or British thermal units per hour (Btu/hr). This rating should be matched to the heating needs for the process fluid at its required temperature.
Expansion tank capacity is the size of the tank used to hold the cooling medium in the system.
Media flow rate is the required flow rate for the process fluid in the system. This rating defines the flow capacity of the pump used in the heat transfer system.
Operating temperature is the accepted temperature or range of temperatures for the heated or cooled process fluids in the system. This is the most essential parameter of the fluid heat transfer system, as it defines whether the equipment fits the temperature needs of application.
Fluid heat transfer systems can use a variety of different media to transfer heat from the heating source to the target system or fluid. Properties to consider include cost, flow characteristics, heat transfer efficiency, and temperature stability/control.
Water is nontoxic and inexpensive, but has a relatively small operating temperature range (low boiling point and a high freezing point). Water is easy to pump but can become corrosive if its pH levels are not maintained properly.
Steam is pressurized water vapor. It provides fast and efficient heat transfer as well as great temperature control, and has no temperature limits.
Hydrocarbon oils have a higher viscosity and lower specific heat than water, but require more energy to pump. They are relatively inexpensive and have low freezing points.
- Synthetic hydrocarbons, also referred to as aromatics, are relatively non-toxic and require little maintenance. They have temperature ranges from -70°F to 750°F.
- Hot oils are petroleum-based fluids with temperature ranges from -10°F to 600°F.
Ethylene glycol and propylene glycol are "antifreezes", meaning they have very low freezing points. Ethylene glycol is extremely toxic and should only be used in a double-walled, closed-loop system. Most glycols deteriorate at very high temperatures. The freezing point, pH value, and concentration of inhibitors must be checked annually to determine whether the mixture needs any adjustments or replacements to maintain its stability and effectiveness. In many applications they are often mixed with water in 60/40 or 50/50 ratios for ideal properties.
Refrigerants such as Dowtherm® and Therminol® are used for medium and low temperature heat transfer applications.
Silicones are silicon-based fluids used in specialized applications requiring material compatibility in the case of leaks. They are not typically cost-effective in comparison to most hot oils and synthetics for similar temperature ranges.
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When requesting a quote for a custom fluid heat transfer system, manufacturers typically request their clients provide the following system specifications:
- The type of fluid being heated
- The maximum operating temperature
- The pressure at the inlet connection between the system and heat transfer device
- The flow rate of the fluid
- The total amount of fluid in the system
Fluid heat transfer systems are also detailed by a number of other specifications relating to the incorporated system, including connections, enclosures, and other features.
Because fluid heat transfer systems typically connect to other equipment or devices, it is important to consider the connections they use. Some fluid heat transfer systems use national pipe thread (NPT) connectors and pipe fittings. Others use 150 lb or 300 lb flanges. Potential buyers must be sure that the heat transfer system being considered has the connections required for the application.
The electronics, control panels, and other environmentally sensitive components of fluid heat transfer systems are typically housed in enclosures rated for specific environments. The National Electrical Manufacturers Association (NEMA), a non-profit trade organization, rates enclosures for many types of electrical equipment.
NEMA 4 enclosures can withstand falling dirt, rain, sleet, snow, windblown dust, splashing water, and hose-directed water.
NEMA 12 enclosures can withstand falling dirt, circulating dust, lint, fibers, and dripping or splashing liquids.
NEMA 7 enclosures are only suitable for indoor use, but in hazardous locations classified as Class I, Division 1, Groups A, B, C, or D by the National Fire Protection Association (NFPA). Class I, Division 1 locations are areas in which volatile flammable liquids or flammable gases are handled, processed or used in potentially explosive amounts during normal operations.
Type Z-purge systems are enclosures that allow fluid heat transfers to be used safely in Class 1, Division 2 environments. Class I, Division 2 locations are areas in which these same materials are used, but confined in secure containers or closed systems.
Type X-purge systems are rated for Class 1, Division 1 environments.
System specifications for fluid heat transfer systems may include a number of features which provide additional functionality or advantages:
- ASME certification for compliance with that organization’s Boiler and Pressure Vessel Code.
- Casters for facilitating and ease of movement.
- Control panels for monitoring and controlling system conditions. Panels may be local or remote.
- Emergency alarms for emergency events such as overtemperature or low coolant levels.
- Indicators or gauges for monitoring system conditions, most notably temperature or pressure.
- Skid mounted for easy lifting, placement, and movement.