Dairy pasteurization is more than heating a product to a target temperature. It is a controlled process in which every particle of product must reach a required temperature and remain there for a specified amount of time.
The heat-transfer system must accomplish this consistently at the design flow rate without damaging the product, creating excessive pressure drop or introducing sanitary risks. Product viscosity, fat content, total solids, fouling behavior and clean-in-place requirements can all affect the design.
Pasteurization Depends on Both Time and Temperature
The time and temperature required for pasteurization depend on the product and process. Under the FDA’s Grade “A” Pasteurized Milk Ordinance, common minimum conditions include:
| Process | Minimum temperature | Minimum holding time |
|---|---|---|
| Batch or vat pasteurization | 145°F | 30 minutes |
| High-temperature, short-time pasteurization | 161°F | 15 seconds |
| Higher-heat, shorter-time processing | 191°F | 1 second |
These minimums do not apply equally to every dairy formulation. For example, the required temperature increases by 5°F when a milk product contains at least 10% fat, at least 18% total solids or added sweeteners. Eggnog and certain other products have separate requirements.
Processors should confirm the required conditions for their product with the current FDA Grade “A” Pasteurized Milk Ordinance, the appropriate regulatory agency, and their process authority.
What Happens in a Continuous Pasteurization System?
A continuous pasteurization system generally includes several thermal stages:
- Preheating or regeneration: Incoming cold product may be preheated using heat recovered from outgoing pasteurized product. Regeneration can reduce both heating and cooling utility requirements.
- Final heating: A heat exchanger raises the product to the required pasteurization temperature using hot water, steam or another heating medium.
- Holding: The product travels through a properly sized holding tube for the required time. Holding time is based on the system’s flow characteristics, not simply the physical length of the tube.
- Flow diversion: If the product does not reach the required temperature, a flow-diversion device prevents it from moving forward as pasteurized product.
- Cooling: After the holding period, the product is cooled to the temperature required for packaging, storage or the next processing step.
The heat exchanger is one part of this system. It must work with the holding tube, flow controls, temperature instrumentation and diversion equipment to maintain a validated process.
Calculating the Required Heat Duty
The starting point for heat exchanger sizing is the required heat duty:
Q = ṁ × Cp × ΔT
Where:
- Q is the required heat-transfer rate
- ṁ is the product mass flow rate
- Cp is the product’s specific heat
- ΔT is the required product temperature change
This calculation identifies how much thermal energy must be transferred, but it does not determine the required surface area by itself. Engineers must also account for the overall heat-transfer coefficient, available temperature difference and exchanger configuration.
In simplified form:
Q = U × A × ΔTlm
Where:
- U is the overall heat-transfer coefficient
- A is the required heat-transfer area
- ΔTlm is the log mean temperature difference
Dairy products rarely behave exactly like water. Higher viscosity, increased solids and product buildup can reduce heat-transfer performance. The exchanger should be sized using product properties at actual operating temperatures and should include a realistic fouling allowance.
Product Characteristics Shape Heat Exchanger Design
Dairy products can vary widely in viscosity, fat content, total solids and sensitivity to heat. These properties affect how readily the product transfers heat and how it moves through a sanitary shell-and-tube heat exchanger.
Important design considerations include:
- Product viscosity across the operating temperature range
- Fat and total-solids content
- Size and concentration of particulates
- Expected product buildup or fouling
- Sensitivity to shear or extended heat exposure
- Required production flow rate and turndown
- Allowable pressure drop
- Clean-in-place requirements
Tube diameter, tube length and the number of passes influence product velocity, heat-transfer performance and pressure drop. Increasing velocity can improve heat transfer and help limit product buildup, but it also raises pressure drop and pumping requirements. Excessive velocity may also be unsuitable for shear-sensitive products.
Clean-in-place conditions should be evaluated separately from production conditions. The exchanger must provide sufficient cleaning-solution flow through all product-contact areas while remaining drainable and free of locations where product could collect.
Because dairy formulations behave differently, heat exchanger sizing should be based on the actual product properties and full range of expected operating conditions.
Sanitary Design Is Part of Thermal Performance
A heat exchanger may meet the required thermal duty and still be unsuitable for dairy service. Product-contact surfaces must also support effective cleaning and help prevent contamination.
Important sanitary design considerations include:
- Product-compatible stainless steel construction
- Appropriate product-contact surface finish
- Smooth, cleanable welds
- Sanitary connections and gasket materials
- Drainable geometry
- Minimal dead legs and product traps
- Access for inspection and maintenance
- Clean-in-place flow and temperature requirements
- Separation between product and utility fluids
- Compliance with applicable 3-A Sanitary Standards
Double tube sheet construction may be specified when additional separation between the product and heating or cooling medium is required. If a tube-to-tube-sheet joint fails, the separated tube sheets can provide a visible leak path rather than allowing the fluids to mix unnoticed.
Information Needed to Design a Dairy Heat Exchanger
Accurate equipment selection begins with accurate process data. Before sizing a sanitary heat exchanger, the manufacturer will typically need:
- Product name and composition
- Minimum, normal and maximum flow rates
- Inlet and required outlet temperatures
- Density and specific heat
- Viscosity across the operating temperature range
- Fat and total-solids content
- Size and concentration of any particulates
- Available heating or cooling medium
- Allowable product-side pressure drop
- Design pressure and temperature
- Expected fouling behavior
- CIP solution, temperature, flow rate and duration
- Required materials, surface finish and sanitary standards
Changes in formulation can materially affect exchanger performance. An exchanger sized for milk may not provide the same results with cream, yogurt mix, ice cream base or another product with higher fat, solids or viscosity.
Engineering the Right Thermal Process
Successful dairy pasteurization depends on matching the exchanger design to the product, production rate, required time-temperature treatment and sanitation program. Proper sizing helps the system reach pasteurization conditions consistently while controlling pressure drop, energy use, fouling and product quality.
Enerquip designs and manufactures sanitary shell and tube heat exchangers for dairy and food-processing applications. Available options include sanitary connections, polished product-contact surfaces, removable tube bundles, and designs built to 3-A.
Contact Enerquip to discuss the product and operating conditions for your dairy heat-transfer application.