FOOD ENGINEERING: HEAT EXCHANGERS FOR LIQUID FOODS
Diagram
Plate and tubular heat exchangers: two shapes for heating liquid food
Plate heat exchanger
Thin liquid flows in narrow gaps between stacked metal plates. Hot liquid flows on one side of each plate and cold liquid on the other.
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Tubular heat exchanger
Liquid flows through tubes, with the heating or cooling fluid flowing around the outside of the tubes.
↓
The choice
Plates suit thin liquids and give fast heat transfer in a small space. Tubes suit thicker liquids and liquids with pieces, and they are easier to clean.
Figure 1: Stacked plates compared with tubes for heating and cooling liquid foods. Drawn in our own style from Karwa, Heat and Mass Transfer, Springer, 2016, Ch. 14, Heat Exchangers, p. 865, and Fellows, Food Processing Technology: Principles and Practice, Woodhead Publishing. Note: the comparison is drawn from the heat exchanger chapters, not from a single head to head passage.
Introduction
A plate heat exchanger and a tubular heat exchanger are the two common kinds of heat exchanger for liquid foods, and both heat or cool a liquid by passing it past a hot or cold surface. In a plate exchanger the liquid flows through narrow gaps between a stack of thin metal plates, with heating fluid on the other side of each plate. In a tubular exchanger the liquid flows through tubes, with heating or cooling fluid flowing around the outside of the tubes.
The core difference is shape and gap: plates give a large surface in a small space with narrow gaps, while tubes give a stronger, wider channel. That difference decides which liquids each type suits, how fast it heats, and how easy it is to take apart and clean.
Quick comparison
Table 1: Plate heat exchanger compared with tubular heat exchanger
| Aspect | Plate heat exchanger | Tubular heat exchanger |
|---|---|---|
| Construction | A stack of thin corrugated metal plates held in a frame | One or more tubes inside a shell, or tubes inside tubes |
| Flow channel | Narrow gaps between plates | Wider round tubes |
| Surface per volume | Very high, so the unit is compact | Lower, so the unit is larger for the same duty |
| Heat transfer | Fast, with high turbulence in the narrow gaps | Slower per unit area, but steady |
| Liquids suited | Thin liquids: milk, juice, thin sauces | Thicker liquids and liquids with pieces or fibres |
| Pressure drop | Higher, because the gaps are narrow | Lower in general |
| Fouling and cleaning | Fouls faster with thick liquids; plates can be taken apart for cleaning | Easier to clean in place, and better for liquids that foul |
| Changing capacity | Easy: add or remove plates | Hard: the tube bundle is fixed |
| Pressure limit | Lower, limited by the plate pack and gaskets | Higher, tubes take high pressure well |
| Food example | Milk pasteurisation in a dairy | Fruit juice with pulp, sauces with pieces, viscous products |
Scientific explanation
Definition
A plate heat exchanger is a heat exchanger made of a stack of thin plates, with the two fluids flowing on opposite sides of each plate. A tubular heat exchanger is a heat exchanger in which one fluid flows inside tubes and the other fluid flows outside the tubes.
Working principle
In both types heat passes from the hot fluid to the cold fluid through a metal wall: by convection from the hot fluid to the wall, by conduction through the wall, and by convection from the wall to the cold fluid. In a plate exchanger the corrugated plates stir the liquid and give a large surface, so heat transfer is fast. In a tubular exchanger the flow in the tubes is steadier, so heat transfer per unit area is lower, but the wide tubes let thick liquids and liquids with pieces pass without blocking.
Apparatus
A plate exchanger is made of corrugated stainless steel plates, gaskets, and a frame that presses the pack together. A tubular exchanger is made of tubes, often in a shell, with pumps to drive the product and the heating or cooling fluid. Fellows and Irudayaraj both describe plate and tubular exchangers for liquid foods.
Measurement procedure
To check either type, the inlet and outlet temperatures of both fluids and the flow rates are recorded, and the heat transferred is worked out from the temperature change and the flow rate. The overall heat transfer coefficient is then found from the heat transferred, the area and the mean temperature difference. The books name no single standard test for this comparison in foods.
Factors affecting readings
The result depends on the flow rates of both fluids, on the viscosity of the product, on fouling on the surfaces, and on the inlet temperatures. Fouling lowers heat transfer over time, and it builds up faster in the narrow gaps of a plate exchanger with a thick or dirty liquid.
Formulas and calculations
The heat transferred in either type follows Q = U x A x delta T lm, where Q is the heat transfer rate, U is the overall heat transfer coefficient, A is the heat transfer area, and delta T lm is the log mean temperature difference between the two fluids.
Worked example: a heat exchanger has U equal to 2000 W per square metre K, area 1 square metre, and a log mean temperature difference of 10 degrees C. Then Q equals 2000 times 1 times 10, which is 20000 W, or 20 kW. A plate exchanger in general reaches a higher U than a tubular exchanger of the same area, so it transfers more heat from the same area. The U value here is an illustration, not a value from the books.
Industrial applications
- Plate heat exchanger: milk pasteurisation and UHT preheating in dairies, where thin milk must be heated fast.
- Plate heat exchanger: fruit juice heating and cooling, where a compact unit suits a thin liquid.
- Tubular heat exchanger: sauces and soups with pieces, where wide tubes will not block.
- Tubular heat exchanger: viscous products such as tomato paste, where a plate gap would foul quickly.
- Tubular heat exchanger: high pressure duties, where tubes take the pressure better than a plate pack.
Advantages and limitations
- Plate heat exchanger, advantage: compact, with fast heat transfer and easy capacity change by adding or removing plates.
- Plate heat exchanger, limitation: narrow gaps foul and block with thick liquids or pieces, and gaskets limit pressure and temperature.
- Tubular heat exchanger, advantage: strong and simple, handles thick liquids and pieces, and is easy to clean in place.
- Tubular heat exchanger, limitation: larger and heavier for the same heat duty, and capacity cannot be changed by adding a plate.
FAQ
Which heats milk faster?
A plate exchanger, because its narrow corrugated gaps give fast heat transfer in a small space.
Why not use plates for everything?
Because thick liquids and liquids with pieces block or foul the narrow gaps between plates.
What is the log mean temperature difference?
The average temperature difference between the two fluids along the exchanger. It is used with U and area to work out the heat transferred.
Can a plate exchanger be taken apart?
Yes. The frame can be opened and plates removed for cleaning or to change the capacity.
Which type is used for tomato paste?
A tubular exchanger in general, because the wide tubes pass a viscous product without blocking.
References and further reading
- Fellows, P., Food Processing Technology: Principles and Practice, Woodhead Publishing (FTN book on disk).
- Irudayaraj, J. (ed.), Food Processing Operations: Modeling, Design and Analysis, Marcel Dekker (FTN book on disk).
- Karwa, R., Heat and Mass Transfer, Springer, 2016 (Google Books page read: Ch. 14, Heat Exchangers, p. 865).
- Incropera, F. P., Fundamentals of Heat and Mass Transfer, Wiley, 2009 (Google Books page read: Heat Exchangers chapter).
