Heating of production halls with heat integration
- 1 GENERAL INFORMATION
- 2 DESCRIPTION OF TECHNOLOGY, TECHNIQUES AND METHODS
- 2.1 HEAT RECOVERY FROM THE PROCESS
- 2.2 WASTE HEAT RECOVERY FROM THE COMBUSTION GASES
- 2.3 WASTE HEAT RECOVERY FROM THE FLUIDS INVOLVED IN THE INDUSTRIAL PROCESS
- 2.4 HEAT RECOVERY FROM WASTE HEAT IN THE AIR CONDITIONING OF WORKSPACES
- 2.5 HEAT RECOVERY FROM THE COOLING SYSTEM COLD FOCUS (AIR HEATING WITH AIR FROM CONDENSERS OR PREVIOUS HEAT EXCHANGE)
- 3 CHANGES IN THE PROCESS
- 4 ENERGY SAVINGS POTENTIAL
- 5 CHANGE IN THE ENERGY DISTRIBUTION SYSTEMS
- 6 REFERENCES
- 7 Case studies
Within an industry, being from the food and drink sector o from other sectors, exist different final energy uses for the heat production. The main consumption is the productive process itself. However, one should not neglect the energy consumption used in the workspaces heating.
The energy demand required for the workspaces heating depends on the same factors that also affects building energy demand:
- Architectural design and constructive materials
- Comfort and healthy criteria
- Internal gains as persons, equipment and lighting.
In the case of industries, not that the health and comfort requirements, and internal gains due to the process are the main factors affecting energy demand (heating or cooling). Productive process with a large amount of heat emission reduces the heating demand for the building acclimatization.In the other hand, spaces with large air renovation requirements due to pollutants, presents high heating energy demands.
In the food and drink industry, it is also important that the quality of the produced food is not affected in the process for heating, cooling or ventilation.
Not all the industries requires heating energy in the productive process. Nor all industries requires additional heat generation for heating spaces, as workspaces or offices. In industries were both are required, two situations in the halls heating may arise:
- Heat energy supply from the same heating generators that feeds the process.
- Heat energy supply from specific heat generators independents of the process.
In an industrial process, heat can be classified according to the fluid temperature:
- Low temperature up to 60ºC
- Medium temperature from 60 to 80ºC
- High temperature above 80ºC
HEATING OF PRODUCTION HALLS FROM THE INDUSTRIAL PROCESS HEAT
The energy levels required in the heating of production halls uses to be lowers than required in productive process, making it possible to use part of the produced heat in the process for the air conditioning. Although full recovery is expected or desired in a process to process application, recovery heat for a process-to-comfort application must be modulated to prevent over heating the make up air .
The use of part of the heat generated for the process can be done in two ways:
- Taking part of the heated fluid that outcomes from the process heat generator.
- Using the wasted heat after the fluid has passed through the industrial process.
Heating at process temperature
This heating method uses the boiler or other heating generators for producing hot water at a process temperature that will feed both the process and the space conditioning. The hot water at process temperature passes through a heat exchanger that heat up the water in the distribution system.
- A single heat production unit
- Less space requirements
- Less variations in the energy supply (if applicable)
- Easy distribution system
- Lower initial investment (in certain cases)
- Inefficiency in the heat production for air conditioning.
- Higher heat production costs
- Higher energy distribution losses due to the higher temperature and the heat exchanger efficiency.
Heating with waste heat
Depending on the process temperature, it is possible to use the waste heat from the fluids that already has been involved in the process and transfer this waste heat to the distribution system for air conditioning. The unique characteristics of the different industrial sector and its production process will draw the lines of the heat recovery, if it is possible, or if it is enough to cover the heating demand, or if it is also required an auxiliary system to cover the whole heat demand. The recovery heat techniques are widely diverse, depending on the fluids, but usually implies the use of a heat exchanger.
The use of waste heat in the air conditioning of the production halls means large energy savings.
- Energy savings
- Money savings and best payback periods for heat production machinery
- Sometimes is difficult to implement energy recovery techniques and equipment in an already built industry. Furthermore if the industry is already in operation.
- The heated liquid may not be a clean liquid so it makes more difficult the heat exchange and needs more auxiliary power in order to pump it through the system.
- Greater complexity in the distribution system for the air conditioning.
Some typical applications for air to air energy recovery are: 
- Flue stacks
- Paint exhaust
- Welding exhausts
HEATING APART FROM HEATING PROCESS
When the heat produced for the process cannot be used for the air conditioning of the production halls (because any reason) and there is heating needs, it is necessary to install specific air conditioning equipment.
The variety of air conditioning systems is very rich and is only limited by terms of comfort, design and characteristics of the production process.
Classification by range
A single unit or a connected and modular package that supplies hot water to a main distribution system does the heat production. From this primary distribution circuit, many secondary loops can be supplied.
The heat production is splat up in different units that supplies different distribution circuit or directly supplies the thermal zone.
Classification by fluid
In those heat-cool cycles that uses condenser and evaporator units, the common fluid is a coolant, with different thermal characteristics than water. This system configuration implies that the primary fresh air renovation have to be carried out by another system. In comparison with aire and even water, coolant distribution systems are of less diameter and needs less space. In the other hand, leaks are very troublesome.
When the system is air distributed, it is common to combine the heated air for the air conditioning and the fresh outdoor air for ventilation requirements. These two flows are mixed and treated in Air Handling Units, that regulates the proportion of air depending on the specific requirements.
Distribution is done by water. Hot water is distributed to the terminal units. In the terminal units heat passes from the water to the air with a heat exchange usually using coils.
Distribution and emission is done with water without any heat exchange with air. Thermal diffusion is done with radiative terminal units. With this kind of systems, an auxiliary ventilation system is needed.
DESCRIPTION OF TECHNOLOGY, TECHNIQUES AND METHODS
Heat recovery in the heating of production halls or other workspaces can be achieved by:
- Waste heat recovery from the thermal process of production.
- Heat recovery from waste heat in the air conditioning of workspaces.
- Heat recovery from the cooling system cold focus (air heating with air from condensers or previous heat exchange).
HEAT RECOVERY FROM THE PROCESS
Heat recovery from the industrial thermal process can be performed in different ways:
- Heat recovery from the combustion gases
- Heat recovery from the fluids involved in the industrial process
WASTE HEAT RECOVERY FROM THE COMBUSTION GASES
Usually called economizers the heat recovery systems that uses the exhaust waste heat to pre heat the water that incomes in the combustion process. For every 22ºC reduction in the gas temperature there is 1% saving fuel in the boiler .
Graphic 1: (a) Coiled tube economizer, (b) rectangular economizer, and (c) cylindrical economizer
Air recovery or air heaters
The air heaters are usually used for taking the sensible heat from the exhaust gases to pre-heat the incoming air in the combustion process. These systems do not use to be combined with air conditioning systems due to the high temperature level in the gases and the difference with the needed temperature level for air conditioning.
Possibilities of heat integration in an industrial process are high, but this kind of systems are commonly used in the equipment itself or for pre-heating the air or water of the main process. Physical placement is important due to minimize the heat loses in transport.
Graphic 2: Air Preheater 
WASTE HEAT RECOVERY FROM THE FLUIDS INVOLVED IN THE INDUSTRIAL PROCESS
Depending on the fluid state and its temperature at the end of the productive process, recovery and thermal exchange methods are different. The main variable that influence the heat exchange is the state of the fluid and its composition (clean or not clean). There are also important variables as the temperature, pressure, thermal shocks, and so on. Many of the industrial process where it is possible to exchange heat are cleaning process with a dirty fluid at the end of the process, charged with particles and other fluids (oils), making more difficult the pass through the heat exchangers.
When the heat fluid is air, most common systems are the heat recovery wheels, plate exchangers and tubular exchangers. When hot fluid is liquid, most common systems are liquid-liquid, letting the liquid-air heat exchangers only for a few applications.
There is a third option: using heat pumps recovery system. This equipment allows the heat exchange between fluids before the incoming to the heat pump cycle. This ways it is possible to reach the desired temperature with less energy consumption.
Hot Air recovery in industrial processes
Rotary wheel heat exchanger
Rotary wheel heat exchanger is a circular honeycomb matrix that rotates. The cold airflow passes through on part of the wheel, and the hot airflow passes through the other part. Configuration, construction and sealing of the wheel do not allow to mix the flows. As the wheel is rotating, the only thermal contact of the flows occurs in the matrix, which is heat absorbent. This matrix can be coated with hygroscopic coatings or be made of porous synthetic fibre to allow and enhanced the humidity adsorption and release, transferring not only sensible but latent heat to the air flows. This kind of wheels are named enthalpy or desiccant wheels.
In the heat exchange process a little pressure drop occurs. The heat rotary exchangers efficiency is about 60-85%  . The higher values correspond to the wheels that also recovers latent heat from air humidity. These machines are quite big, because of the air ducts and the wheel itself, so it is necessary to take into account enough space for its installation.
Graphic 3: Rotatory Wheel heat exchanger 
Plate Heat Exchangers
Plate heat exchangers are systems that separates two air flows with solid elements (plates). The heat exchange occurs through the plates, that can be built with fins to maximize the contact surface and increase the exchange efficiency. Depending on the flows direction the system will be parallel flows or countercurrent flows. The plate heat exchanger efficiency is around 75% .
Graphic 4: Plate and Plate-Fin heat exchangers 
Tubular heat exchangers
Tubular heat exchangers are systems that uses the natural convection as the physical phenomenon for the air movement. Tubular heat exchangers are built as a duct with a liquid layer inside. In one side of the duct, a heat flow passes through the exchangers, and heats up the walls of the duct and the liquid inside. The liquid evaporates and starts to move inside the duct. In the other side of the duct, a cold flow passes through the exchanger. In this side, the vapour that have moved on from the hot side get cold, transferring its heat to the exchanger duct walls, and it condensates. This way the tubular heat exchanger heats up the initially cold air.
Graphic 5: Tubular heatexchanger 
The air-air heat exchangers have the inconvenience of the air ducts, much bigger than the water or liquid ones have to be placed side by side, and this imposes physical limitations. These heat exchangers, with the desiccant wheel exception, do not recover latent heat.
The liquid-liquid heat exchangers do not need such a big place as the air-air heat exchangers. The liquid ducts are narrower and losses are lower. The auxiliary consumption is also lower.
Hot liquid recovery in industrial processes
Tubular heat exchangers
Tubular heat exchangers are devices built with two concentric ducts. Hot fluid and cold fluid passes along the ducts without mixing. Usually these heat exchangers has an U shape, so they are also known as hairpin heat exchangers.
Fluids flows in countercurrent. Usually are used for small application with high pressure and temperature. The inner tube can be built with a smooth surface duct (less exchange efficiency) or as different finned tubes (more exchange efficiency).
This type of devices are recommended when fluids are dirty and carry solid particles or sludge that can make difficult the flowing through to narrow ducts. Furthermore, the drop pressure is low and are thermal shocks and vibration proof.
Graphic 6: Tubular Heat Exchangers (from the RheoHeat website: http://www.rheoheat.se/b15_heat.html)
Condensates Heat recovery
In the process where vapour is used a hot carrier fluid, or in those where vapour is generated because of one of the processes, it is possible to recover the latent heat from gas to liquid status change. Taking the latent heat, the result will be saturated water (liquid phase) at a high temperature, nearly the boiling temperature (variable due to the pressure) and latent heat could be used for an energy exchange with other process, either a production process or for conditioning workspaces.
HEAT RECOVERY FROM WASTE HEAT IN THE AIR CONDITIONING OF WORKSPACES
Workspaces in industry are regulated by demanding ventilation normatives, focused on maintaining low pollutant levels that can affect the health of employees. When ventilation requirements are high due to health issues and large amount of fresh air is impulse inside the workspace at an ambient temperature, heat thermal demand increases. In case that workspaces needs to be conditioned, pre-heating and heat recovery technniques for the air flow become important.
If it is impossible to recover heat from the process (for any reason), there are also other possibilities that allows the reduction of the thermal demand.
- Air pre-heating with buried ducts
- Air pre-heating in Air Handling Units.
Air pre-heating with buried ducts
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 hotter ground (in winter) increases the air temperature. This temperature rise depends on the soil type, ambient temperature, ducts length and airflow speed.
The pre-heated air in the buried ducts is conducted then to an air handling unit to clean it and if it is necessary to raise its temperature to the design desired value with additional coil heating.
This system present as the main disadvantages the initial construction investment, the required space and the thermal limitations of the energy exchange.
Air pre-heating in Air Handling Units
Exhaust air from the workspace, with a temperature near the comfort temperature can be conducted back to the air-handling unit to be exchanged with the air fresh supply. The air exhaust conditions will limit if the exchange can be direct or indirect due to the pollutants presence. Depending on the temperature achieved, it is possible to do not require additional heating to condition the incoming air. Sensors and the control unit will decide in each moment the openness of the gates in the AHU to regulate the airflow.
The heat exchangers usually are configured as a counter current heat flow between the incoming and outcoming of fresh air.
This device has as a main disadvantage the over investment on the exhaust air system. Air now has to be conducted to the AHU, and it means additional ducts and fans. Depending on the pollutants in the exhaust air, this air cannot be directly mixed with fresh air, and the exchange has to be indirect, so the exchange efficiency will be lower.
Enthalpy recovery system
Enthalpy recovery systems are a variation of the concept of heat recovery ventilation in which air flows are not physically mixed but do so through various exchange devices (gates, plates, desiccant wheel, etc.). The main characteristic is that enthalpy recovery systems are also capable to recover the sensible heat and part of the latent heat of the exhaust air.
HEAT RECOVERY FROM THE COOLING SYSTEM COLD FOCUS (AIR HEATING WITH AIR FROM CONDENSERS OR PREVIOUS HEAT EXCHANGE)
A condenser in a refrigerant system is a heat exchanger that eliminate all the heat that has been generated in the cycle. The condenser evacuates the heat generated in the evaporator as well as the heat of the energy input of the compressor. From the compressors outcomes hot and pressurized fluid that have to be cooled in the condenser. The condenser exchanges this heat with a medium: water or air.
This rejected heat can be recovered from the water or air and can be used in the workspace heating.
Most common condensers by water are:
- Shell-and-tube condenser
- Shell-and-coil condenser
- Tube-in tube-condensers
- Brazed-Plate and Plate-and-Frame Condensers
Air-cooled condensers uses ambient are to exchange heat. There are three main coil construction types:
Heat recovery from the condensers it is difficult to implement in an already existing condensers, because most of them are designed to work at an ambient temperature, and there is not much heat to recover then. In order to increase the heat recovery, it is important to increase the temperature of condensation, but this implies to readjust the cooling system to check if the compressors is ready to work at higher temperatures or if the insulating will last as it is expected. The COP of the cooling unit decrease also if the condensation temperature is increased. Actually, cooling plants incorporates recovery systems in the condenser units and have been designed in order to ensure an optimum performance.
Other options for heat recovery in the condensers could be:
- Heat recovery in screw compressors. This compressors uses oil in the process, which absorbs a large amount of heat. Temperatures are high enough to be harness in some applications.
- Heat recovery in ammonia compressors. The ammonia reaches very high temperatures and needs that the compressor has been designed to evacuate all this heat with a water circuit.
There is a third type of condensers that mixes the air and water properties. In an evaporative condenser, high pressure and hot vapor circulates through a coil that is continuously in contact with water and an airflow. This allows the vapor in the coils to reject heat to the water in the outside surface of the coils and heat the water until it vaporization point.
These condensers are more efficient than water or air condensers, and more compact, but works at lower temperature because are limited by wet bulb temperature (always lower than dry bulb temperature).
Heat can be harness from the warm air that has passed through the coils and the water spray curtain.
Evaporative condenser can be more efficient with the installation of a deshuperheating coil. A desuperheater is an air cooled finned coil, usually installed in the discharge airstream of an evaporative condenser . The coil is placed before the wetted condensing coil and it is used to remove the superheat before the vapor enters the evaporative condenser.
The desuperheating coils needs a very high temperature of the incoming fluid to be able to reject all the heat. This is why are limited to work with reciprocating compressor with ammonia working as fluid. The discharge temperatures of this system rea relatively high, between 120ºC to 150ºC.
CHANGES IN THE PROCESS
ENERGY SAVINGS POTENTIAL
CHANGE IN THE ENERGY DISTRIBUTION SYSTEMS
 HVAC System and Equipment. 2012 ASHRAE Handbook.
 HVAC Systems and Equipment. 2012 ASHRAE Handbook.
 Heat Exchange handbook. KuppanThulukkanam
 Heat Exchange handbook. KuppanThulukkanam
 Appendix D: Improving Energy Efficiency in Power Plant Operation. G.G. Rajan.
 Fundamentals of Heating Systems. ASHRAE.
 Fundamentals of Heating Systems. ASHRAE.
 Fundamentals of Heating Systems. ASHRAE.
 Fundamentals of Heating Systems. ASHRAE
Fundamentals of Heating Systems. ASHRAE
HVAC System and Equipment. 2012 ASHRAE Handbook.