Cooking & boiling with heat integration

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GENERAL INFORMATION

Best Application Techniques in the Food, Drink and Milk Industries, June 2005.

Cooling and boiling are heat processing techniques applied to foodstuffs, principally to make the food edible. They also alter the texture, colour and moisture content of the food and may facilitate other later processes.

Cooking and boiling are applied on an industrial scale for the preparation of ready –to-eat meals, in the preparation of complete meals and for meal components, such as in various meat products. They are also applied to heat foodstuffs prior to final processing.

Cooking is carried out in ovens. There are several different types of ovens; water bath, shower, steam hot air and microwave.

• Water bath ovens allow the best homogeneity of heating to be obtained. The immersion into hot water causes weight loss, and results in proteins and greases being released into the water. Boiling is cooking in a water bath oven at boiling temperature.
• Shower ovens allow a good uniformity of heating. They include the simultaneous action of water sent through the showers and the saturated steam, which rises from the collecting basin, at the bottom of the oven, which is heated, in turn, by coils.
• In steam ovens, the water shower is eliminated and any heating is only due to the steam produced in the collection basin.
• Hot air ovens include, when it is necessary to control surface humidity, a steam inlet and a recirculation of hot air, which is obtained by passage through heat exchangers.
• In a microwave oven, food is heated by passing microwaves through it; the resulting generation of heat inside the food facilitates rapid cooking.

U.S. Department of Energy, Energy Efficiency and Renewable Energy.

Waste gas heat are unavoidable in the operation of all fuel-fired furnaces: boilers, oven, dryers and so on. In these devices, fuel and air are mixed and burned to deliver heat to the process. When combustion is at the maximal temperature, the flue gases must be removed to allow the entering of fresh air. In this process, a lot of thermal energy is wasted in the exhaust gases. Much energy is lost through the furnace also:

• Heat storage in the furnace structure
• Losses from the furnace outside walls or structure
• Heat transported out of the furnace by the load conveyors, fixtures, trays, etc.
• Radiation losses from openings, hot exposed parts, etc.
• Heat carried by the cold air infiltration into the furnace.
• Heat carried by the excess air used in the burners.

DESCRIPTION OF TECHNOLOGY, TECHNIQUES AND METHODS

In hot air ovens, the most common recovery system is a heat exchanger into the chimney that exchanges energy with the flue gases from the ovens. However, this heat exchanger creates a drop pressure and cool down the gases. This effect reduces the draught in the chimney and the oven will need more fuel. It is very important then to calibrate correctly savings from the heat exchanger and the excess of fuel consumption. There is the possibility of installing a variable frequency fan that controls the speed and draught in the chimney measuring the flue gases pressure drop and regulating the fan speed for the optimum operational point.

The temperatures of the cooking and boiling process allows in many cases the production of vapour after the heat exchanger. It is possible also to implement in a secondary phase another heat exchanger for hot water production.

Reducing exhaust loses should be always the first step when looking for efficiency in fuel fired systems. There are also some techniques to increasing energy efficiency through reduction in exhaust gas heat loses:

• Minimize exhaust gas temperatures
• Minimize exhaust gas volumes
• Use of oxygen enriched combustion air
• Use of oxygen enriched combustion air

Once exhaust loses are in its minimum, it is time to recover the wasted heat. Great efficiency improvements can be made even on well-calibrated systems. There are four main methods:

DIRECT HEAT RECOVERY TO THE PRODUCT

If exhaust gases can be put in contact with a cool incoming load, thermal energy will be transferred to the load and preheats that load. This would reduce the energy that escapes with the exhausts and the direct contact allows a high efficiency of the thermal exchange.

AIR HEATERS

Air heaters preheats the air that is entering in the combustion chamber. An air heater is a gas-to-gas heat exchanger, placed on the stack of furnaces. Depending on the necessity there are a widely range of designs available, but most commons are tubes or plates heat exchangers. The exhaust gases and the incoming air are not mixed. This are the most common recovery devices.

REGENERATORS

Regenerators are heat storage devices, which are rechargeable. Exhaust gases passes through the regenerator device, an insulated container filled with metal or ceramic, and heats it up. When it comes to the maximum temperature, exhaust gases shuts off. Then the fresh cold air that is going to enter to the combustion chamber passes through the regenerator and it heats up until the regenerator is thermally empty.

For a continuous operation, two systems with two regenerators are necessary to be able to change the flow when the regenerators need to be charged.

USE OF WASTE HEAT BOILER

A waste heat boiler is a boiler that works without combustion of fuel and air. The water is heated up with the exhaust gases that escapes from the previous burner. The use of a waste boiler needs to be well calculated, in order to be able to feed the boiler with the correct amount of gases volume and temperature. This boiler would only be operational when the previous burn process is running.

Hot water from water bath ovens or shower can be also suitable for being exchanged with a cold fluid entering the oven or another thermal process. Best heat exchangers for this application are tubular exchangers, and in general, all kind of exchangers that performs well with dirty hot fluid. After the process, the hot water that has to enter into the heat exchanger is dirty with oil and fats, so it is necessary to be contemplated in the heat exchanger selection.

CHANGE IN THE ENERGY DISTRIBUTION SYSTEMS

• Case studies
  • Berger
  • Gösser Brauerei

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