FOOD ENGINEERING: HEAT TRANSFER IN FOODS
Diagram
Natural and forced convection: what moves the fluid
Natural convection
Warm fluid rises and cool fluid sinks on its own, because warm fluid is lighter. No fan or pump is needed.
↓
Forced convection
A fan, pump or stirrer pushes the fluid past the surface, so the fluid moves much faster than it would on its own.
↓
The result
Forced convection gives a higher heat transfer coefficient, so the surface heats or cools faster than in natural convection.
Figure 1: Fluid moved by density differences compared with fluid moved by a fan or pump. Drawn in our own style from Karwa, Heat and Mass Transfer, Springer, 2016, and Incropera, Fundamentals of Heat and Mass Transfer, Wiley, 2009.
Introduction
Natural convection and forced convection are the two kinds of convection heating, divided by what makes the fluid move. In natural convection, also called free convection, the fluid moves on its own: warm fluid becomes lighter and rises, while cool fluid sinks, and that slow circulation carries heat. In forced convection a fan, pump or stirrer pushes the fluid, so it moves fast past the surface.
The core difference is the driving force: density differences in natural convection, and a machine in forced convection. Forced convection moves heat faster, because fast moving fluid brings fresh hot or cold fluid to the surface all the time, while natural convection is gentle and slow.
Quick comparison
Table 1: Natural convection compared with forced convection
| Aspect | Natural convection | Forced convection |
|---|---|---|
| What moves the fluid | Density differences: warm fluid rises, cool fluid sinks | A fan, pump or stirrer |
| Fluid speed | Slow, set by the temperature difference | Fast, set by the fan or pump |
| Heat transfer coefficient | Low | High, several times higher |
| Heating or cooling speed | Slow | Fast |
| Equipment needed | None, the fluid moves on its own | A fan, pump or agitator, with power and control |
| Energy use | No power for moving the fluid | Power is needed to run the fan or pump |
| Temperature uniformity | Less even, with warm and cool zones | More even, because the fluid is well mixed |
| Control | Hard to control, it follows the temperature difference | Easy to control by changing fan or pump speed |
| Food example | Still air in a store room warming a cold pack slowly | Forced air in a blast chiller cooling food fast |
| When it appears | Still ovens, open tanks, store rooms | Forced air ovens, heat exchangers, chillers |
Scientific explanation
Definition
Natural convection is convection in which fluid motion is caused by density differences that come from temperature differences in the fluid. Forced convection is convection in which fluid motion is caused by an outside means, such as a fan, pump or stirrer.
Working principle
In natural convection, fluid near a hot surface warms, expands, becomes lighter and rises, while cooler fluid moves in to take its place, so a slow circulation forms. In forced convection the fan or pump sets the speed, and the fast stream sweeps heat away from the surface or brings heat to it, so the thin layer of slow fluid at the surface is thinner and heat passes faster. Karwa gives empirical relations for each kind in separate chapters, because the relations use different numbers: forced convection uses the Reynolds number, and natural convection uses the Grashof number.
Apparatus
The equipment for forced convection is a fan, blower, pump or agitator. Natural convection needs no moving equipment. In both cases thermocouples at the surface and in the fluid, and a meter for fluid speed, are used to check the heating or cooling.
Measurement procedure
To compare the two, the surface temperature and the fluid temperature are recorded over time while the food heats or cools, first in still fluid and then with the fan or pump running. The heat transfer coefficient is then worked out from Newton’s law of cooling. The books name no single standard test for this comparison in foods.
Factors affecting readings
Natural convection depends on the temperature difference between surface and fluid, on the size and shape of the surface, and on whether the surface faces up, down or sideways. Forced convection depends on the fluid speed, on the fluid properties, and on the shape of the surface. In both cases the properties of the fluid change with temperature, so the readings change as the food warms or cools.
Formulas and calculations
Both kinds follow Newton’s law of cooling: q = h x A x (Ts – Tinf), where h is the heat transfer coefficient. The difference is in the value of h: the books give empirical relations for h, with forced convection relations in one chapter and natural convection relations in another, and no single short formula covers both.
Worked illustration, using simple numbers to show the arithmetic only: a surface of area 0.01 square metres is 50 degrees C hotter than the fluid. In still air, with h equal to 5 W per square metre K, q equals 5 times 0.01 times 50, which is 2.5 W. With a fan, with h equal to 50 W per square metre K, q equals 50 times 0.01 times 50, which is 25 W, or ten times more. The h values are illustrative, not values from the books.
Industrial applications
- Natural convection: still air store rooms, where food warms or cools slowly without fans.
- Natural convection: open tanks and kettles, where hot liquid rises and circulates on its own.
- Forced convection: blast chillers and freezers, where fast cold air cools food quickly and evenly.
- Forced convection: forced air ovens, where a fan circulates hot air so bread bakes evenly.
- Forced convection: heat exchangers, where a pump drives the liquid food past the hot or cold wall.
Advantages and limitations
- Natural convection, advantage: simple and free to run, with no fan, pump or power for moving the fluid.
- Natural convection, limitation: slow and uneven, so heating and cooling take a long time.
- Forced convection, advantage: fast and even heating and cooling, and easy to control by changing the fan or pump speed.
- Forced convection, limitation: needs equipment, power and maintenance, and fast air can dry the surface of food.
FAQ
Why does warm air rise?
Because warm air is lighter than cool air, so it floats up while cool air sinks. That slow movement is natural convection.
Which kind cools food faster?
Forced convection, because the fan brings fresh cold air to the surface all the time.
What is the Grashof number?
A number used in natural convection relations. It describes how strongly density differences drive the flow, in the same way the Reynolds number describes forced flow.
Can both kinds act at once?
Yes. In a fan oven with the fan off, natural convection still acts. With the fan on, forced convection dominates.
Why does a blast chiller use a fan?
To force cold air fast past the food, which raises the heat transfer coefficient and cools the food quickly and evenly.
References and further reading
- Fellows, P., Food Processing Technology: Principles and Practice, Woodhead Publishing (FTN book on disk).
- Karwa, R., Heat and Mass Transfer, Springer, 2016 (Google Books page read: Ch. 8, Empirical Relations for Forced Convection Heat Transfer, p. 539, and Ch. 9, Empirical Relations for Natural or Free Convection, p. 623).
- Incropera, F. P., Fundamentals of Heat and Mass Transfer, Wiley, 2009 (Google Books page read: free and forced convection sections).
