A Different Perspective
Author: David Greenplate, Tek-Temp Instruments
Source From: Process Heating Magazine
Posted Date: 2009-11-01
Liquid-to-liquid heat exchangers offer an option to air-cooled chillers for some process cooling applications.
Consider the following questions:
- Is the tap or building cooling water source compromising the cooling performance of your process equipment due to its lack of cleanliness?
- Is the required cooling temperature to your process at or above ambient?
- Do you require precise temperature control for your process?
- Are there situations in which your cooling water source temperature is too low?
- Has your process equipment been damaged due to high cooling water pressure?
If you answered yes to any of these questions, you should seriously consider a liquid-to-liquid heat exchanger system. The following will discuss the overall features and options available to these systems.
Your tap or building water can have changes in temperature, pressure and quality, and all of these can make it incompatible with your process equipment. Liquid-to-liquid heat exchange systems differ from standard air-cooled chillers: they are non-refrigerated and use a tap or building water source to remove heat from your process. These systems separate the process coolant fluid and the building water to transfer the heat. They can produce precise temperature control for processes that do not need temperatures below the tap or building water source temperature. An internal plate heat exchanger is used to remove heat efficiently while isolating the cooling and process loops.
The non-refrigerated heat exchange systems utilize a tap or building water source to remove heat from the process.
An internal plate-type heat exchanger is used to remove heat efficiently while isolating the cooling and process loops. Large quantities of heat can be removed in a small physical space. Liquid-to-liquid heat exchangers are particularly useful in situations where the cooling water source temperature is too low, or where the pressure and cleanliness of the water may cause damage to the equipment or compromise performance.
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Large quantities of heat can be removed in a small physical space. Although changes in the building water source due to the season of the year may affect the overall cooling capacity, in general, heat-removal capacities are dependent upon the cooling water flow rate or the process water flow rate. If there is a noticeable decrease in the system’s cooling performance, the heat exchanger should be checked for sludge or scale in the internal heat exchanger. By following the manufacturer’s maintenance procedures, the heat exchanger will run well over its performance time.
Precise Temperature Control. These systems will supply precise amounts of temperature-controlled coolant to your process equipment. The coolant fluid temperature is constant despite fluctuations in the facility water temperature and flow rate. The temperature controller varies the tap or building water flow rate through a motorized valve and then through the heat exchanger to achieve the desired process water temperature. In many cases, for more precise temperature control, an RTD may be used with a microprocessor-based temperature controller.
Energy Efficiency. Because the coolant fluid loop is completely separated from the tap or building water, it keeps your process equipment free from contaminants. The tap or building water is conserved due to the motorized valve varying the flow. These systems also are energy efficient. The pump, motor and controller are the only pieces requiring power. Because they do not have fans or compressors, they are quiet and do not exhaust hot air into the room as standard air-cooled chillers do.
Heating. If the application calls for precise high temperature control, heating can be added. This is done most commonly with immersion heaters but for greater heatup time, recirculating cartridge heaters can be used. Immersion heaters are directly immersed in the water/glycol mixture within the system’s reservoir tank. These heaters are essentially 100 percent energy efficient because they are generating all the heat within the process fluid. The heating capacity can vary depending on the specific application. Often, with the heating option, the manufacturer will include a dual-stage level switch and an overtemperature switch as standard.
Backflow Prevention. Backflow prevention can be installed on these systems as well. This option is vital when the heat exchanger is located at a height below the process. Because the reservoir is not sealed, when the system is turned off, the fluid in the lines will flow back to the reservoir, causing the tank to overflow. With backflow prevention, a check valve is installed on the supply line and a solenoid valve is installed on the return line. The check valve will allow fluid to flow to the process, but it prevents it from flowing back. Conversely, the solenoid allows fluid to flow when the system is on but closes when the system shuts off.
Coolant Filters. Because in a liquid-to-liquid heat exchanger there are no condenser fins as with a standard air-cooled chiller, an air filter is not needed. However, a process coolant filter may be an important option to consider for protecting the equipment. Particulates in the fluid can cause damage to the equipment or, in some cases, build up so much that it causes a blockage in the coolant lines. The filter element should be checked and replaced periodically to keep the system at top performance.
So, when the time comes to choose new temperature control equipment for your process, check into a liquid-to-liquid heat exchanger. It may be the right choice for your process cooling application.
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