Dairy processing: Heat exchangers
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Introduction
The following provides recommended practices for free-standing heat exchangers that are not part of the pasteurization system and that are used to heat or cool dairy products.
Heat exchangers are widely used in the dairy processing industry for fore-warming or cooling the milk product and for cooling cheese brine. They are designed to transfer heat from a hot liquid to a cold liquid. Heat exchangers can cause the final product to become contaminated if they are not properly controlled.
- Monitor heat exchanger equipment regularly under an equipment maintenance program to minimize contamination risks
Types of heat exchangers
Most heat exchangers are one of the following types.
Spiral type: These are tubular type preheaters which are fully enclosed inside the tube chests or inside milk vapour areas. These systems are impossible to inspect inside, therefore a cleaning program is essential.
Surface cooler type: In this system milk is distributed over the cooling coils by means of a distributor pipe or trough and the milk drops from the lower coil into a collecting trough. Product is exposed to air in this system therefore, subjecting the product to air-borne contamination.
Tubular type: These heat exchangers consist of a small tube mounted concentrically within a larger tube. Product is pumped through the inner tubes, entering at the low point and leaving at the high point while the heating or cooling medium enters the larger tube at the high point and flows through the space between tubes, leaving at the bottom. The system is designed to keep the two mediums separate.
- Monitor and record the pressure daily to ensure that the pressure is higher on the milk product side than the medium side by at least 14 kPa (2 psi)
- Cleaning without daily dismantling is an accepted practice when an effective cleaning program is established
Plate type: These heat exchangers consist of a number of stainless steel plates, sealed at the edges with gaskets, and are clamped tightly within a press. The spaces between the plates are occupied alternatively by milk and the heating or cooling medium. The system is designed to keep the two mediums separate.
- Monitor and record the pressure to ensure that the pressure is higher on the milk product side than the medium side by at least 14 kPa (2 psi)
- Establish a routine program to monitor the condition of plates (pin holes in plates, gasket condition, cracks, etc.), taking into consideration the design specifications, operating conditions and hours of operation, wear and the history of the plates and gaskets
- If pin holes are found in any plate in any section, check all plates in the same section
- If inspection reveals that the plates are dirty, inspect and hand clean the exchanger daily until an effective cleaning program is established
- Check the integrity of all food contact heat exchange surfaces at least once per year (for example by dye recirculation, dye check, pressure retention, etc.)
- If there are problems with heat exchanger integrity (plate or gasket issues), implement a more frequent inspection program to verify that the problem has been remedied
- Keep records to show testing has occurred
- Also document the cause of any failure (for example age, compression, metal fatigue, etc.)
Additional controls for pasteurized products
If the product passing through the heat exchanger is pasteurized finished product, an automated mechanism is an effective means of achieving the correct pressure relationship in the cooling section during forward flow, divert and shutdown conditions, so that the pressure on the cooling media side is not greater than that of the pasteurized product at all times.
- Where there is an automatic mechanism, ensure the cooling medium supply is stopped or diverted and the cooling medium side is vented to atmosphere in the following cases:
- During forward flow, when the product pressure on the pasteurized product side drops to within 2 psi of the cooling medium side of the plates and
- During diverted flow and shutdown conditions
- If a product vacuum breaker is in use, ensure the venting of the cooling medium side is at an elevation below that of the product vacuum breaker
- Where there is not an automated mechanism and the product passing through the heat exchanger is pasteurized finished product, have documented control measures to ensure the correct pressure relationship is maintained:
- Monitor and record the pressures a minimum of twice daily during production, at beginning and end of run
- Check the cooling media for microbiological contamination (for example psychrotrophs, coliforms) at a frequency of at least once per week
- Test the pH of the cooling media at a frequency of at least once per week
- Visually check the cooling media at least once per week
- Conduct pinhole tests and plate teardowns at a minimum of once every six months
- Have scheduled plate replacement
- Examples where an automatic mechanism or written program is not necessary include:
- The product being cooled is raw product which will ultimately be pasteurized
- The potable water being used as the coolant is non-recirculated
- Tubular type heat exchangers are used
- Use water treatment compounds in the cooling media that are safe and suitable for dairy processing
- This is particularly important when there is no automated mechanism
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