FLOWNEX : one-stop-tool for HVAC system design

 

As the name implies, the Heating Ventilation and Air Conditioning (HVAC) industry is focused on three central functions namely heating, ventilating and air-conditioning. These are interrelated in providing thermal comfort and acceptable indoor air quality within reasonable installation, operation, and maintenance costs. Additional to these functions, control plays an important role in maintaining the required HVAC conditions. HVAC systems can also reduce air infiltration, maintain pressure relationships between spaces, humidify/dehumidify environments as well as control air quality with regards to gasses and other contaminants.

HVAC considerations forms an important part in the design of small, medium and large residential and industrial buildings as well as other specialised systems such as aircraft and marine environments where safe and healthy environmental conditions are regulated with respect to temperature, pressure, humidity and air quality. The HVAC industry relies heavily on thermal fluid systems to operate, making Flownex ideal for design, optimisation and simulation in this industry on component, system and complete product level. Although interrelated, for the purpose of this document, typical Flownex applications are presented in terms of heating, ventilation, air conditioning, integration and specialised systems sections.

Heating

Heating systems may form part of a complete HVAC system or may only serve the single purpose of heating. Heating systems are typically installed as single local heating units or as complete central heating systems. Local heating systems typically comprise of a single unit used to heat a single room by means of electricity or gas. Typical examples are:

  • baseboard heaters
  • space heaters
  • radiant heaters
  • wall heaters
  • furnaces (fireplace)
  • heat pumps

Central heating systems use a single (or in bigger installations multiple) heat source located at a central location from where the heat is distributed throughout the system to the “consumption” points. Typical systems are circulation systems and forced air systems.

Circulation systems (hydronic systems)

Circulating systems (hydronic systems) uses water, steam or oils to transfer heat from the central heating source to the “consumption” point. The heating of the circulation fluid is typically done in a type of heat exchanger such as a boiler where coal, natural gas or other fuels are burned. Different types of boilers with varying efficiencies are used such as conventional and condensing boilers. Alternatively if a district heating system is employed, the fluid may be directly circulated through the central heating system or heat transferred via a heat exchanger to the central heating systems’ circulation fluid.

Circulation in the system is achieved by means of a pump which circulates the fluid through the system. In steam systems no pump is required for circulation purposes. The circulation fluid is carried from the heating point to the consumption points by means of a distribution network. The network typically comprises of pipes, isolation valves, bleed valves, automatic bypass valves, thermostatic control valves, feed and expansion tanks and radiators. Systems utilising steam may also include steam traps and pressure relieve valves. The radiators are heat exchangers in which heat is transferred from the circulation fluid to the air within the heated environment. Distribution networks may also comprise of a number of tubes passing within the floor, therefore heating the floor and the room accordingly. The heated circulation fluid may also be used for other secondary uses such as heating water in water heaters for domestic use.

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Fig 1 – Schematic of a hydronic central heating system

Flownex can be used for the simulation and system design of circulation central heating systems. Its ability to simulate the temperature dependence of oil’s (and other fluids’) viscosity and thermal capacity enables the user to simulate and design the required heating and pumping capacity of central heating systems. Flownex fundamentally solves the flow distribution in multi parallel flow paths, thus enabling system design to optimise distribution network layout for optimum circulation fluid distribution. Flownex has advanced heat transfer capabilities taking into consideration the thermal mass of the circulating fluid as well as that of the distribution network material. Flownex heat transfer capabilities also include conduction and convection heat transfer as is typically found in radiators and boilers. Heat loss to the surroundings for various insulating materials and thicknesses is also calculated for the distribution network.

Flownex has two phase fluid capabilities, enabling the simulation and design of steam systems where condensation and boiling occurs. The Flownex designer can be used to optimise the complete system and the Flownex control library used to simulate and design the systems’ control philosophies and control components. Apart from Flownex’s ability to simulate central heating systems, it can further be used in the simulation and design of district heating systems on a much larger scale, encompassing all the phenomena encountered in central heating systems.

Forced Air Systems

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Fig 2 – Schematic of a central forced air heating system.

In a forced-air system, air is used as the heat transfer medium. These systems rely on ductwork, vents, and plenums for air distribution. The system uses a central air handler to heat the air. The central air handler comprises of a filtering system which, depending on the required air quality, uses different methods to “clean” the air. Heat exchangers/coils are used to heat the air, either by direct methods such as electrical resistance heaters, gas firing and heat pumps or indirect heating methods such as through hot water or steam from a boiler or district system. The air handler can also employ humidification devices to humidify the air which may become dehumidified as a result of continuous heating. These systems may have a mixing chamber where fresh outside air is introduced into the cycle and may also serve to regulate the air supply temperature via dampers. These systems use a fan (blower) to provide the required pressure differential to drive the flow through the system. On larger air handler systems, heat recovery devices may be installed to extract heat from the returning air which is discarded and transfer it to the fresh incoming air. From the central air handler, the heated air is fed to the supply plenum from where the air is directed to the rooms which the system is designed to heat. Returning air is collected via return vents and ducted to the central air handler for re-heating. Thermostats are used to control the temperature and distribution of the forced air heating systems.

Flownex can be used for the simulation and system design of forced air central heating systems. Its ability to determine pressure loss and flow distribution through multiple parallel flow paths enables the user to design the system layout and component sizes, damper placement as well as blower sizing. Flownex has advanced heat transfer capabilities taking into consideration the thermal mass of the circulating fluid as well as that of the distribution network material. Flownex heat transfer capabilities also include conduction and convection heat transfer as is typically found in air handler units. Heat loss to the surroundings for various insulating materials and thicknesses is also calculated for the distribution network.

Flownex has the ability to simulate air and water vapour mixtures, enabling the user to determine the relative humidity throughout the system as well as condensation and evaporation phenomena. This also enables the user to simulate the effect of humidifier/dehumidifiers and their effect on the relative humidity. The Flownex designer can be used to optimise the complete system and the Flownex control library used to simulate and design the systems control philosophies and control components.

Ventilation

Ventilation is the process of replacing air to improve indoor air quality by means of controlling temperature, replenishing oxygen as well as removing/introducing moisture to control humidity levels. Additionally ventilation systems also serve to remove and control excessive moisture, odours, smoke, heat, dust, airborne bacteria, and carbon dioxide as well as to keep building interior air circulating thus preventing stagnation of the air.

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Fig 3 – Schematic of a natural draft ventilation systems


Methods for ventilating a building (or other spaces) may be divided into natural ventilation and mechanical or forced ventilation. With natural ventilation a building is ventilated with fresh outside air without the use of a fan or other mechanical system. This can be achieved with open able windows, trickle vents and other architectural systems where warm air in the building can be allowed to rise (stack effect) and flow out of the building thus forcing cool outside air to be drawn into the building naturally through openings in lower areas of the building. These systems use minimal or no energy but may not be able to maintain thermal comfort in warmer humid climates. Figure 3 shows a schematic of a natural draft ventilation system.

Another form of natural ventilation is the solar chimney. A solar chimney is a structure which is exposed to the sun (solar irradiation), thereby heating the chimney and accordingly the air inside the chimney. The less dense warmer air in the chimney rises, forcing colder air in at the bottom (buoyancy driven flows). The solar chimney can be used to ventilate buildings or rooms, drawing in fresh air from the outside through windows or ventilation openings. Alternatively fresh air can be drawn in through a geothermal heat exchanger or evaporative cooler (for example passive downdraft cooling tower), thereby further cooling the incoming air. Figure 4 shows a schematic of a solar chimney incorporated into a house design. Ventilation air is drawn in through a geothermal heat exchange process, thereby aiding to keep the buildings’ temperature constant.

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Fig 4 – Schematic of a solar chimney and geothermal heat exchange.

Mechanical or forced ventilation utilizes fans and other mechanical means to ventilate spaces by means of extraction or pressurizing the ventilated environments. Ventilation can be focused on ventilating the complete building or alternatively on specific high “contaminant” areas such as local exhaust ventilation. Processes such as welding, machining and chemical processes may require the removal of “contaminants” at the point of origin. This may be done by focusing the extraction hood at the source and extracting the contaminant by means of mechanical or pressurized ventilation.
To prevent “sick building syndrome”, local and international standards exist, specifying the ventilation rate. The ventilation rate is normally expressed by the flow rate of fresh outside air being introduced into the ventilated building. These may typically be based on a volume flow rate per minute on a per person or per unit floor area basis or can alternatively be expressed as the number of air changes per hour.
Ventilation system layout is dependent on type and the function they have to perform. These systems typically comprise of a distribution and/or extraction system consisting typically of fans, ducting, air control components (dampers), distribution/collection vents, filters as well as control and monitoring systems.

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Fig 5 – Schematic of a mechanical or forced ventilation system.

Flownex can be used for the simulation and system design of ventilation systems. Flownex has advanced heat transfer capabilities taking into consideration the thermal mass and buoyancy effects of warmer less dense air, circulating due to buoyancy effects in solar chimney and natural ventilation systems. Its ability to determine pressure loss and flow distribution through multiple parallel flow paths enables the user to design the system layout, component sizes, damper placement, filter sizes as well as to perform blower sizing for mechanically ventilated systems. Flownex has the ability to simulate air and water vapor mixtures, enabling the user to determine the relative humidity throughout the system as well as condensation and evaporation phenomena and where it may occur. The Flownex designer can be used to optimize the complete system by means of designing for user specified parameters (for example maximum duct velocity or humidity). The control library can be used to simulate and design the systems’ control philosophies, control components and monitoring systems as well as to determine their optimal positions in the system.

Air Conditioning

Air conditioning may be described as the process of altering the air temperature, humidity and quality to more favorable conditions. Air conditioning in essence is done by any form of cooling, heating, ventilation or “cleaning” that modifies the condition of the treated air or environment. Since air conditioning is interrelated to heating and ventilation processes as discussed above, this section will focus specifically on:

  • Refrigeration cycle (Vapour compression cycle)
  • Humidification and dehumidification
  • Evaporative cooling

Refrigeration cycle (heat pump)

In a typical refrigeration cycle, a heat pump is used to transfer heat from a low temperature heat source to a high temperature heat sink. This operation is opposite to natural heat flow which occurs from a high temperature heat source to a low temperature heat sink.

In a typical refrigeration cycle, the refrigerants’ liquid/gas phase change at different pressures (and therefore different temperatures) is used to transfer latent heat at a constant temperature to and from the air. In the evaporator refrigerant, at a low pressure (and therefore low temperature), evaporates and therefore absorbs heat from the primary air stream, thereby cooling it. The evaporated refrigerant is compressed by means of a compressor (energy input is required for the compression process) to a high pressure (and therefore high temperature). The high pressure refrigerant gas is condensed in the condenser and therefore discards heat to the secondary air stream, thereby heating it. Depending on the heat flow direction required, the refrigeration cycle can either be used for heating or cooling of the conditioned area.

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Fig 6 – Schematic of a typical refrigeration cycle.

Refrigerant in the evaporator heat exchanger may be at a lower temperature than the air’s dew point temperature. This will result in vapour condensation, and therefore a reduction in the airs’ humidity. The combination of cooling the air and condensing water vapour from the air can be used effectively to control the conditioned area’s temperature and humidity, therefore providing favourable conditions according to requirements. Single stand-alone units can be used for small areas. Larger refrigeration systems may be incorporated into ventilation systems where the refrigeration system can switch between heating or cooling mode, depending on the conditioned areas’ requirement and the systems’ control philosophy.

Changes in condenser/evaporator side air temperature have an impact on the efficiency of heat pumps. In geothermal heat pumps, the condenser/evaporator side (depending on cooling or heating mode) is connected to a circulation loop buried underground at a depth where the temperature is more or less equal to the regions’ yearly average temperature. The circulation loop may employ water, water-glycol mixtures or other heat transfer fluid which is circulated through the system by means of a circulation pump and transfers heat to or from the ground. In turn, the circulation loop exchanges heat with the heat pump by means of a heat exchanger, thereby providing the heat pump with a fairly constant condenser/evaporator temperature. This results in increased heat pump efficiency when compared to air circulating units.

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Fig 7 – Schematic layout of a geothermal heat pump used for residential heating/cooling.

Flownex can be used for the simulation and system design of refrigeration systems. Its ability to simulate single phase flow (gas and liquid) as well as two phase flow (gas and liquid mixture) enables Flownex to simulate fluid conditions through the complete cycle. Heat transfer in the evaporator and condenser can be simulated during the phase change. Flownex has the ability to simulate the heat exchanger material resistance as well as the thermal mass of the system which can aid in the design of secondary circulation loops as is encountered in geothermal heat pumps. Pressure losses and flow rates can be simulated for the heat exchanger’s (air and refrigerant sides), enabling the user to perform component sizing. Flownex has the ability to simulate air and water vapour mixtures, enabling the user to determine the temperature and the relative humidity through the system as well as condensation and evaporation phenomena and where it may occur. The Flownex designer can be used to optimise the complete system by means of designing for user specified parameters. The control library can be used to simulate and design the systems’ control philosophies, control components and monitoring systems.

Dehumidification and humidification systems

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Fig 8 – Dehumidifier using the refrigeration cycle principle

Humidification and dehumidification devices are used to regulate the humidity of conditioned spaces. In areas of high humidity and temperature, it is often necessary to reduce the humidity levels in order to increase the comfort levels in conditioned areas. On the other hand, in colder less humid areas, humidification may be required to create a comfortable environment for the occupants in the conditioned area.

Dehumidification is typically done by means of passing humid air over a cooling cool having a temperature lower than the air’s dew point temperature. Water vapour in the air condenses and flows from the cooling cools. The air is then reheated to its original dry bulb temperature, but now has less water vapour and therefore a lower humidity. The cooling and reheating processes may be done by means of passing cold and hot water through tubes or alternatively heating by an electrical heater. A method commonly used is to place both the evaporator and condenser coils of a refrigeration cycle in the same air stream which is to be dehumidified. The evaporator coil is placed first, thereby cooling the air, causing condensation. The air is then passed through the condenser coils where it is reheated. Due to the added energy input from the compressor, this type of dehumidifier will have a higher air outlet temperature when compared to the air inlet temperature, therefore also acting as a heater. Dehumidifiers may be used as stand-alone systems, used to dehumidify single rooms or small areas. Alternatively they may be used in conjunction with large HVAC systems for dehumidification of complete buildings.

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Fig 9 – Ultrasonic humidifier, utilizing a piezo-electric transducer to generate water mist.

Humidification is done by adding water vapour to the air, thereby increasing the humidity of the air. This can be done by means of boiling water, vibrating piezo-electric transducers, vaporizing spray nozzles, as well as wick and pad systems increasing the air water evaporation interface. In essence these systems increase the air-water interface area, thereby allowing the water to evaporate, effectively increasing the air’s humidity. Humidification devices can be used as single stand-alone systems, used to humidify a single room or small areas as well as larger units incorporated into HVAC systems used to condition large building or home environments.

Flownex can be used for the simulation and system design of humidification and dehumidification systems. Apart from the design of dehumidifiers, the actual humidity distribution throughout conditioned areas can be determined by means of Flownex. This enables the user to determine the required humidification/ dehumidification to be conducted as well as the placement of these devices in larger systems. The effects of changing environmental conditions on the humidity and distribution thereof throughout the system can also be determined. The control library can be used to simulate and design the systems’ control philosophies, control components and monitoring systems as well as to determine the correct placement of measurement equipment.

Evaporative Cooler

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Fig 10 – Evaporative cooler

Evaporative cooling uses the latent heat of vaporization of water to cool air by means of evaporating water. This process is employed in an evaporative cooler and is especially suited for warm dry climates. In these dry weather conditions, the evaporative effect may also increases the humidity of the air to more comfortable conditions.

A typical evaporative cooler comprises of a fan, drawing air through a wetted medium. The wetted medium increases the air-water contact area, thereby increasing the amount of water evaporated and therefore the amount of cooling achieved by means of evaporation. Water is continuously circulated through the wetted medium to ensure that it is kept wet at all times. As water is evaporated by the system, it needs to be replenished with fresh water to ensure that the system does not dry out.

Flownex can be used for the system design of evaporative cooling systems. As with humidification and dehumidification devices, Flownex can be used to determine the temperature and humidity distribution through ventilation systems employing evaporative coolers. The design and component selection of the evaporative cooler can be optimised using Flownex designer software. The effect of changing ambient conditions on the evaporative coolers’ performance can be simulated and the resultant effect on the complete ventilation system determined.

HVAC system integration, fault detection and training

HVAC system integration

Although heating, ventilation and air conditioning is discussed as separate sections above, these are typically integrated into a complete HVAC system, comprising of all the above mentioned sections. Since all of these systems have an influence on each other and ultimately perform functions to provide thermal comfort and acceptable indoor air quality, the integration of the individual systems needs to be optimised to provide a complete HVAC system performing optimally and according to the requirements.

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Fig 11 – Schematic of a building employing a complete HVAC system to provide thermal comfort and acceptable indoor air quality to the occupants.

Flownex can be used for the simulation and system integration design of complete HVAC systems. Flownex has the ability to handle multi fluid networks, enabling the simulation of refrigeration cycles, district heating systems as well as the conditioned air and water vapour cycles in a single complete network representing the HVAC system. This allows the user the ability to determine the integrated performance and the effect that the individual systems have on each other, thereby aiding in component sizing, selection and HVAC system optimisation. The Flownex designer and optimiser capabilities can be used to design the system for specific parameters while other parameters are optimised to reduce cost and increase performance. Flownex has transient solution capabilities, allowing the user the ability to determine the HVAC systems’ performance with changing ambient conditions as well as to determine the systems’ response to changing user thermal set-points and individual sub systems performance variation. The Flownex control library can be used to simulate control philosophies and measurement positions of existing HVAC systems as well as to design new control and measurement systems for new and existing HVAC systems.

Fault detection and training

In large HVAC installations, system and component faults such as leaking valves, stuck dampers and underperforming subsystems can go undetected due to the systems’ control compensating for the fault. These errors may result in increased power consumption and even component or system degradation, resulting in increased energy cost bills and costly HVAC system repairs.

Flownex can be used as a fault detection tool. Comparing the actual HVAC systems’ performance to the Flownex models’ performance prediction on a regularly basis, can effectively point out possible faults. Depending on the level of detail the Flownex model is simulating, the model can also be used to point out possible areas to focus on during fault finding, reducing maintenance cost and system downtime.

The Flownex model of an existing HVAC system can additionally be adapted to act as a training simulator for operating and maintenance personnel. Such a model can be used for operator training on larger HVAC systems without affecting the actual plant. Additionally it can be used to investigate the effect on the complete system of component downtime due to maintenance.

Concluding

In general, Flownex is well suited for the design and simulation of thermal hydraulic networks such as those encountered in the HVAC industry. Its standard library of components applicable to HVAC industry networks (pumps, fans, valves, dampers, pipes, air ducts, custom losses, heat exchangers, tanks, heat transfer elements, compressors etc.) can easily be configured to resemble existing and/or design new thermal fluid networks. Additional to these, a series of fundamental components such as convective, radiative and conductive heat transfer components as well as custom pressure loss and other custom components exist which can be used to simulate components fundamentally.

Flownex has an extensive library of fluids applicable to the HVAC industry. It enables the user to create gas mixtures as well as customised fluids to allow tracking of gasses in a HVAC system. The two phase fluid library has an extensive range of refrigerants, allowing the user the capability to simulate refrigeration cycles. The fluid library also has air and water vapour fluid mixtures, allowing the calculation of dry and wet bulb temperatures, humidity as well as evaporation and condensation processes throughout the HVAC system.

Flownex has a control and electrical library, thus enabling the user to set up complete integrated networks of the whole system. The Flownex designer capability allows the user to design for multiple parameters in a single designer run. This allows the user to design for a number of design specifications (such as maximum duct velocity and humidity) in a single simulation.

In conjunction with the engineering model, a Graphical User Interface (GUI) can easily be set up, enabling HVAC system optimisation and fault finding as well as operator training for operational personnel of large HVAC systems. The Flownex solver allows fast simulation speeds (in the order of real time solution, depending on network size), thus enabling fast and on site simulation and decision making.

 

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