Cooling of production halls with heat integration
- 1 GENERAL INFORMATION
- 2 DESCRIPTION OF TECHNOLOGY, TECHNIQUES AND METHODS
- 3 CHANGES IN THE PROCESS
- 4 ENERGY SAVINGS POTENTIAL
- 5 CHANGE IN THE ENERGY DISTRIBUTION SYSTEMS
- 6 CASE STUDIES
- 7 REFERENCES
The food industry has large energy consumptions for cold production. In these industries, the end use of cold can be used in:
- Productive process
- Food preservation
- Air conditioning of workspaces
Between these end uses the most important could be the food preservation due to cold chambers are operating continuously.
Air conditioning of workspaces is wide variable, it depends on the food industry branch, and the food needs for preservation. Another essential factor to take into account is the weather of industry placement, because cold demand for preservation and productive process in south of Spain can be very different from in Germany.
Energy demand required for cooling workspaces is influenced too by similar variables that affects a cooling building demand:
- Architectural design and constructive materials
- Comfort and healthy conditions
- Internal gains (persons, equipment and lighting)
In the industrial sector, the main variables that affects the cooling demand are the comfort, healthy conditions and the heat gains due to the process.
Industries with production process with high heat gains generation have high cooling energy demands. As explained before, cooling demands are produced in:
- Food preservation
- Air conditioning of workspaces
Cooling and preservation of food, very common in the food industry, is solved usually with cold chamber units, that works as a unique unit, with an electricity consumption and its own cold circuit, but without a distribution system that allows the energy (cold or heat) integration within the process. Because of this, once installed it is difficult to develop a cold integration between cold chambers and other process facilities.
The air conditioning of the workspaces where food is manipulated uses to be at low temperatures, and because of the presence of persons and other heat gains, has a high cool demand. It is very common also that these rooms are not sealed from outdoor air, so the mass exchange with exterior air heats up the indoor air. The air conditioning of these workspaces can be done with the same equipment that is used to supply the offices (depending on the temperatures). In the small and middle industry, it is common a high dispersion of the facilities, that usually do not work together and serves specifically end uses.
The air conditioning in office spaces is focused to offset the internal gains of people, lighting and equipment in the office. In the climatic regions in south of Europe, due to the warm temperatures and a greater solar radiation, cooling demand in offices may not be underestimated.
In the workspaces heating, there are much more opportunities in heat integration due to many productive processes that needs heat and the possibilities of exchanging energy are high. However, the situation in cold integration is different because cooling needs in process (apart from food preservation) are lower, and in many cases, cold is produced and applied in the machine itself.With different cold generators for the different end uses, the recovery cold systems are used separately.
Cold generation from wasted heat is possible if desired temperatures are not too low. Absorption and adsorption techniques are suitable to supply cold for comfort end use as the halls or workspaces air conditioning. However, cold processes may need lower temperatures.
DESCRIPTION OF TECHNOLOGY, TECHNIQUES AND METHODS
The technologies described here are not really energy integration technologies. As it has been said before, cold process integration are much more difficult than heat recovery. Technologies described below are focused in take advantage from the weather conditions for cooling the air, and reducing this way the cooling demand.
The buried pipes systems works in the same way than for heating, but in the opposite direction.
Ground temperature oscillates during all the year, as the air temperature does, but in a lower amplitude. Therefore, in the cold months, ground temperature is some degrees higher than air temperature, while in summer time, ground temperature is some degrees lower than air temperatures (over all during the day).
Soil temperature not only depends on the weather but also on the depth at which it is measured. The deeper the measurement point is, the less influence of air temperature and therefore less variability throughout the year and the greater the variation between air temperature and soil temperature.
The buried pipes systems allows to harness the thermal ground inertia, and its lower temperature amplitude during the year to pre-heat air in winter and pre-cool in summer time.
Outdoor fresh air passes circulates through the buried ducts, usually built in concrete. The heat exchange with a coldest ground (in winter) decreases the air temperature. This temperature drop depends on the soil type, ambient temperature, ducts length and airflow speed.
The pre-cooled air in the buried ducts is conducted then to an air handling unit to clean it and if it is necessary to low its temperature to the design desired value with additional coil cooling.
This system present as the main disadvantages the initial construction investment, the required space and the thermal limitations of the energy exchange.
The free-cooling technique is used, as the buried pipes, with the purposes of pre-cooling the incoming outdoor fresh air that enters the air-handling units. Depending on the air temperature after the process, is possible to do not need an additional cool load from the cooling generation unit.
The free cooling consist in the addition of gates in the air-handling units that allows the fresh air to be conducted directly to the inside of the building without passing through the cold coils. The control unit compares the temperature between the outside air and the inside air and if it is inside the desired range, actuates over the gates. Air temperature sensors are needed in order to collect the information and send it to the control unit.
This system can be operated during all day (and night), with the unit control and the sensors, as it has been said before. It is important to remark that the system should be well calibrated in order to not allow fresh air at an ambient temperature when it is not required. Another way to operate the system is only during the night. Depending on the schedules in the industry, if there is no process neither occupants during night, it is possible to simplify the control unit of the AHU device, to allow only the direct fresh air income during night (less needs of checking temperatures). This method suits best on daily schedules.
The evaporative cooling is based on a different cooling cycle than the conventional cool cycle that uses the compressors and evaporators. The evaporative cooling bases its operating principle on harnessing the needed latent heat of a fluid, as the water, to be evaporated.
By circulating hot and dry air through a water layer, for example, a spray curtain, water particles absorb the heat in the air, transforming it into latent heat of vaporization and cooling the air.
This system does not need energy consumption devices as the compressors. The main electric consumption is the impulse and exhaust of the air. Because of this, the final energy consumption and the environmental cost is low. However, it is important to identify and study the air conditioning design parameters in the zone, because the amount of heat that can be taken from the air is a function of its temperature and relative humidity. In the zones, where temperatures below the evaporation system capacity are required it is possible to add some cooling coils that allows the needed temperature drop.
This system requires a continuous water supply, so it is recommended for locations with plenty of water and a hot and dry weather.
This system is not very recommended to be installed as and stand-alone system if there is plenty of cooling loads in the workspace (as it happens in the offices). It is much recommended although in workspaces where the purpose is just to refresh the incoming air reducing its temperature some degrees, in order to ensure a better thermal comfort.
COLD PRODUCTION FROM WASTE HEAT
An absorption chiller produces chilled water by transferring heat from the chilled water circuit to the re-cooling circuit (closed cycle). The absorption cycle is driven by heat supplied to the generator.
The main components of an absorption chiller are evaporator, condenser, generator and absorber as well as expansion devices and a solvent pump. For HVAC applications and chilled water production, water and Lithium bromide are most commonly used as refrigerant and solvent, respectively. Absorption chillers are available as single-effect, double-effect or triple-effect machines. Double- or triple-effect chillers can be applied when high driving temperatures are available and lead to higher COPs compared to single-effect absorption chillers.
Graphic 1: Hydraulic scheme of a single-effect absorption chiller (H2O-LiBr) for water cooling; re-cooled with water (example temperatures) 
Heat comes from a heat generator or in order to integrate the heating process, can be taken from the exhaust liquids or gases of the productive process.
Table 1: Characteristics data of absorption chillers for the production of chilled water and process cooling. 
An adsorption chiller (closed process) produces chilled water driven by heat as an absorption chiller, but makes use of a solid absorbent instead of a fluid solvent.
The main components of an adsorption chiller are evaporator, condenser and receiver/generator as well as an expansion device. For example, water and silica gel can be used as refrigerant and adsorbent, respectively. As the solid adsorbent cannot be pumped, at least two beds are required which alternately work as receiver and generator for obtaining a relatively continuous cooling production. The chiller contains automatic butterfly valves for changing between the chambers. 
Graphic 2: Hydraulic scheme of a two-bed adsorption chiller for water cooling; re-cooled with water 
CHANGES IN THE PROCESS
ENERGY SAVINGS POTENTIAL
CHANGE IN THE ENERGY DISTRIBUTION SYSTEMS
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