WASTE HEAT RECOVERY POTENTIAL FOR HVAC SYSTEMS
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TABLE OF CONTENTS
1.0
EXECUTIVE SUMMARY ......................................................................................................................................................... 3
2.0
INTRODUCTION...................................................................................................................................................................... 5
3.0
SETTING THE SCENE ............................................................................................................................................................. 6
4.0
AIR-TO AIR HEAT RECOVERY OPTIONS .......................................................................................................................... 9
5.0
SPECIFIC NUMBERS............................................................................................................................................................. 15
6.0
AT DESIGN OR RETRO-FIT? ............................................................................................................................................... 17
7.0
WORKED EXAMPLE ............................................................................................................................................................. 18
8.0
REFERENCES .......................................................................................................................................................................... 21
APPENDIX 1............................................................................................................................................................................................. 22 APPENDIX 2............................................................................................................................................................................................. 22
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1.0 EXECUTIVE SUMMARY 1.1
Report Summary
1.1.1
The brief for this report was to carry out a review of the Air-to-Air Heat Recovery technologies available for use in the Commercial and Industrial business sectors. The target audiences are Energy Engineers and those involved in Facilities Management.
1.1.2
The research for the report considered a number of commercially available Air-to-Air Heat Recovery Technologies; with particular focus on ‘Sensible’ heat recovery.
1.1.3
The premise of the report is that where possible the use of air re-circulation is optimized before heat recovery is considered.
1.1.4
The report outlines why Heat Recovery should be considered and presents the potential effects on Capital and Operating costs.
1.1.5
The report presents a list of Technical data to be considered; and a list of information required by the designer at design stage to allow the economic viability of heat recovery to be assessed.
1.1.6
The report describes the various commercially available Heat Recovery technologies and presents a comparison table of the technologies showing typical operating parameters, advantages and disadvantages.
1.1.7
The report outlines factors to be taken into account when considering technical options at Design and Retro-fit stages of a project.
1.1.8
The report presents a worked example using a run-around coil Heat Recovery assembly. The calculation results show the total nett KWh savings, the total nett annual cost savings and the predicted simple Payback Period.
1.1.9
Finally the report presents sample Heat Exchanger selections based on the design conditions used in the worked example.
1.2
Findings/ Conclusions
1.2.1
Following after optimization of air recirculation and control strategies, the potential for Heat Recovery should be considered on every new project, and on existing projects if not already previously considered.
1.2.2
All heat recovery devices add to the system resistance in an air handling installation, and therefore increase operating costs. Heat recovery becomes economically justifiable when the capital investment is recouped by the nett operating cost savings, within a reasonable Payback Period. As a number of variables have to be considered, each situation needs to be assessed to determine if the application of heat recovery is viable.
1.2.3
The four types of Air-to-Air Heat Recovery devices described in this report are suitable for Commercial and Industrial applications; however the particular technical challenges of the project will dictate the most suitable solution, e.g., -
Where cross-contamination is a concern, Run-around coils or Heat pipes are recommended.
-
Where supply and extract air streams are in close proximity – Heat pipes, Plate Heat Exchangers or Rotary Wheels may be used. Where air supply and extract air streams are separated – Run-around coils are the only viable option. This is often the case in retro-fit situations. Plate Heat Exchangers are widely used in Residential and Commercial large and small scale applications. Where Sensible and Latent energy recovery is required – a Rotary Wheel is the most suitable choice. Where the exhaust air stream contains corrosives; suitably treated run-around coils or Plate Heat Exchangers may be used.
-
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1.2.4
Heat pipes are particularly suitable in dehumidification applications, e,g, Swimming Pools
The worked example in section 7.0 shows the Total nett energy savings and the Total annual cost savings for a 5.0 m3/s air handling system (full Fresh air) using a run-around coil heat recovery installation. The simple Payback Period is calculated at 1.2 years. The Table below shows comparative data for the other heat recovery technologies using similar design conditions: Heat Recovery Device
Effectiveness (%)
Capital Cost (€)
Total Annual Cost Saving (€)
Simple Payback Period (Yrs)
Heat Pipe
49
14,500
11,032
1.3
Run-around Coil
49
13,200
10,602
1.2
Rotary Wheel
67
11,000
15,873
0.7
Plate Heat Exchanger
67
10,000
14,499
0.7
Note: The costs indicated are rough order of magnitude figures which include capital cost of equipment, controls, installation and design fees.
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2.0 INTRODUCTION 2.1 2.1.1
2.2 2.2.1
Special Working Groups The Special Working Groups (SWGs) initiative was developed by the Sustainability Energy Authority of Ireland as part of the Energy Agreement Programme. The SWGs focus on particular technologies, initiatives and areas of interest to the Programme members. The focus of this report is on ‘Waste Heat Recovery potential for HVAC systems’. Assumptions and Exceptions The following assumptions and exceptions have been made during the production of this report, a. The report focuses on Air-to-Air Heat Recovery technologies for HVAC applications in Commercial and Industrial buildings. b. A number of technologies are described, and a worked example presented for a system using a ‘Sensible’ (temperature only) Heat Recovery device. c. The report considers standard HVAC operating temperatures and pressures. d. The report does not consider summertime cooling of incoming fresh air using an Energy Recovery device. e. Recirculation of exhaust air is the cheapest and most efficient form of Heat Recovery. Others forms of Heat Recovery should only be considered after the need for fresh air is minimised, and the use of air recirculation and control strategies to optimise energy consumption at air handling plant have been firstly fully exploited.
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3.0 SETTING THE SCENE 3.1
Why Recover Heat?
3.1.1
Recirculation of building extract air is the cheapest and most efficient form of Air-to-Air Heat Recovery since it involves little or no energy penalty. However in certain circumstances, e.g. where the exhaust air contains contaminants such as odours, chemicals, dust or corrosives it may not be possible to re-use this air. In these circumstances air-to-air heat recovery devices should be considered in both new and refurbishment projects.
3.1.2
Any time the temperature of the air being discharged from a building is higher than the incoming air to the building there is a potential opportunity for heat recovery. There is great scope for energy conservation if the heat in the exhaust air can be reclaimed and applied as a source of energy to raise the temperature of the incoming outside air. Heat recovery systems use heat energy that would otherwise be rejected as waste to pre-heat the incoming air, resulting in saved energy, lower running costs and potentially reduced plant capacities.
3.1.3
A variety of devices are available which facilitate air-to-air heat exchange, these include, • • • •
Run-around coils (Water circulation) ‘Heat Pipe’ Heat Exchangers Air-to-Air Plate Heat Exchangers Rotary Air-to-Air Heat Exchangers
Two of the above devices deliver direct air-to-air heat exchange and two employ an intermediate circulating medium. The actual level of heat recovery will depend on the type of heat recovery device selected and the temperature difference between the supply and extract air streams. All heat recovery devices create a resistance against which a fan has to operate (pressure drop). This causes the fan to work harder to maintain flow rate, so increasing electricity consumption. Where an intermediate circulating fluid is used there is the additional consideration of the electrical consumption of the circulating pump and the losses from the interconnecting pipework. The amount of energy saved by installing a heat recovery device is equal to the energy recovered less the extra energy used in operating pumps, fans, etc. The final decision on installing heat recovery systems depends on economic viability. As the cost of electricity is greater than the cost of fossil fuels, the heat recovery device will need to recover enough energy to economically justify its inclusion, while delivering a reasonable Payback Period.
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3.2 3.2.1
Potential effects on Capital and Operating Costs The following table outlines the potential effects on Capital and Operating costs associated with the installation of a Heat Recovery system. Expenditure
Capital Costs
Savings
- Design Costs
- Reduction in Central plant size
- Supply and Installation costs of the completed assembly
- Reduction in distribution pipework
- Modifications to existing plant and services to accommodate the new equipment - Additional plantroom space - Controls
Running Costs
- Additional operating costs due to increased fan power.
- Nett reduction in operating costs due to heat recovery.
- Pump operating costs where runaround coils are employed.
- Reduced Carbon emissions
- Increased maintenance
3.3 3.3.1
Technical items to be considered (at design stage and when considering a retro-fit) The following technical items should be considered when selecting an Air-to-Air Heat Recovery device, a. b. c. d. e. f.
Space accommodation requirements of the Heat Recovery System. Distance between the supply and extract air streams. Type of Energy recovery required (‘Sensible’ only or ‘Total’ Energy (Sensible and Latent)) Supply and Extract air quantities (mass flowrates) to be accommodated. ‘Effectiveness’ of the Heat Recovery device. Quality and condition requirements of the Supply Air stream (Is cross-contamination acceptable? – consider the risk of cross contamination between exhaust air streams and supply air streams due to Carry-over or Leakage). g. Quality and condition of the Exhaust Air stream (Corrosive, dust laden, High temperature, High static pressure), leading to additional costs for anti-corrosion coatings, additional filtration, robust construction, etc. h. Construction materials – consider corrosion, location, differential pressures, contaminants, etc. i. Additional operating energy and service requirements for the Heat Recovery system, e.g. electrical supplies and condensate drains. j. Modification requirements to existing plant and service routes to accommodate the new equipment. k. Move-in space provision, i.e. consider the move-in path for a large item of equipment. l. Construction costs. m. Disruption to existing occupied areas, e.g. downtime. n. Additional maintenance costs – filtration, cleaning, motor drives, etc. o. Impact on the performance of existing air handling equipment, e.g. additional pressure drop resistance on existing fans. p. Control method (also, additional controls and upgrading of existing BMS systems). q. Condensation formation and frosting on the exhaust air side of the heat exchanger. r. Maximum allowable pressure drop through the unit (particularly when the unit is being retrofitted to an existing system). s. Air flow arrangements (Plate Heat Exchangers – Cross-flow or Counter-flow) t. Face velocities at the Heat Exchanger.
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3.4 3.4.1
3.4.2
Economic Justification Assuming the technical requirements can be met, the final decision on the installation of a heat recovery device will be made on the grounds of its economic justification. This justification requires an assessment of the potential energy savings in comparison to the system capital cost and the increased operating costs. In order to carry out an assessment the following information is required, a. b. c. d. e. f. g. h. i.
The operating period for the equipment - annual hours run. The outside air conditions (temperature and hours of occurrence) for the heat recovery season – Local Weather Data. The extract air temperature. The supply and extract air volume flowrates. The ‘Effectiveness’ (efficiency) of the heat recovery device being considered. Effectiveness quoted in manufacturer’s data is usually based on a balanced air flow, i.e. equal supply and extract air volumes. The cost of Electricity and Fossil Fuels. The boiler efficiency Supply and Extract Fan efficiencies (also include circulating pump efficiencies where a runaround coil assembly is being considered) The installed cost of the selected heat recovery equipment (including design costs, controls costs, pipework costs (run-around coils), electrical costs and the cost of any necessary modification to existing services).
The economic investment is considered justified if the assessment shows the capital cost investment can be recovered by the energy cost savings related to the installation of the heat recovery system, within a reasonable Payback Period. A reasonable Payback Period would be considered to be of the order of 1- 6 years for most businesses. 3.4.3
The viability of heat recovery increases when: a. b. c. d.
The number of air changes per hour increases and the heating season lengthens. The temperature difference between the supply and extract air streams increases. The supply and extract air streams are within close proximity (though run-around coils can be considered where this is not the case). The system operates for extended periods of time, e.g. a system which operates 24/7 will yield higher savings than a system which operates 10/5.
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4.0 AIR-TO AIR HEAT RECOVERY OPTIONS 4.1
System Descriptions
4.1.1
The following describes commonly available Heat Recovery technologies,
4.1.2
Run-around coils (Water circulation)
Figure 1 – Run-around coils The run-around coil arrangement is essentially a ‘Sensible’ heat transfer device. Finned air-towater heat exchangers are installed in the ducts between which the heat is to be transferred. A closed loop, counter-flow water or water/glycol (for freeze protection) circuit is used to transfer heat from the warm extract air to the cooler supply air. An expansion tank is required to allow fluid expansion and contraction. A three-port temperature control valve allows regulation of the circuit and prevents the exhaust coil from freezing. The valve is controlled to maintain the temperature of the fluid entering the exhaust coil at 5ºC or above. This condition is maintained by by-passing some of the fluid around the supply air coil. The valve can also ensure that a prescribed air temperature from the supply air coil is not exceeded. The small temperature differences attainable in HVAC applications will result in deep coils (typically 6-8 rows). Overall heat transfer efficiencies are relatively low given the two stage heat transfer process (exhaust air to fluid & fluid to supply air). Electrical energy costs required to drive the circulating pump and overcome the additional coil pressure drops (Supply air coil and Exhaust air coil), and maintenance costs need to be taken into account. A significant advantage of this arrangement is its flexibility – Supply and Exhaust ductwork do not need to be in close proximity, coils may be fitted to any number of exhaust ducts and the heat collected and distributed to any number of similar supply air ducts. Diversity of energy availability and energy demand between air handling plants may thus be used to best advantage. The system is particularly useful where cross contamination between the exhaust air stream and the supply air stream is a concern (e.g. Laboratory application), since the arrangement does not involve close contact of air streams. Modulation control can be achieved by pump operation an/or bypass valve arrangements on the coils. A 25% (by mass) inhibited glycol mixture is often used in the circulating pipework to provide frost protection of the circuit. The concentration of glycol affects the specific heat capacity of the circulating fluid and the efficiency of the heat exchanger. The greater the concentration of glycol, the lower the specific heat capacity, and the lower the heat transfer efficiency. Coil heat recovery loops use coils that are constructed to suit their environment and operating conditions. To ensure optimum operation of the coils the air streams should be filtered. Anticorrosion protection of the exhaust air coil may be provided when the exhaust coil is placed in a corrosive air stream.
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Main Features: Materials of construction: • Tube Material – Copper, Aluminium and Stainless Steel • Fin Material - Copper, Aluminium and Stainless Steel Performance: • Effectiveness: 50 – 65% (Sensible) • Resistance to air flow: Approx. 150 – 500 Pa Typical Applications: • General HVAC systems in Commercial or Industrial applications where ‘sensible’ heat recovery is required. • Retrofit HVAC applications where plant space is limited and/or supply and extract ductwork are not in close proximity. • Where cross-contamination of air streams in not permitted.
4.1.3
‘Heat Pipe’ Heat Exchanger
Figure 2 – ‘Heat Pipe’ Heat Exchanger The ‘Heat Pipe’ heat exchanger is essentially a ‘Sensible’ heat transfer device. The Heat Pipe has its origins in the nuclear industry and was further developed as part of the space programme. The device is a passive heat exchanger of which there are two main types – vertical and horizontal. Finned tubes are mounted in banks in a similar manner to a cooling coil. Supply and extract sections are separated by a dividing partition. A working fluid is employed to effect heat transfer. The fluid used is selected to suit the temperature range required and would typically be one of the common refrigerants. The heat exchanger is made up of rows of conductors. Each individual conductor is a sealed tube, pressure and vacuum tight, provided with an internal wick of woven glass fibre normally as a concentric lining to the tube. In operation, the heat applied in the evaporation section (exhaust air duct) will cause the fluid to vaporise and travel to the condensation section (supply air duct) of the pipe. As the vapour condenses the heat is transferred and the vapour returns to a fluid, thus completing the cycle. When mounted in the horizontal orientation, capacity control of the pipe is achieved by adjusting the horizontal angle of the heat pipe about the centre of its base. This is achieved through the use Page 10 of 29
of a tilting mechanism fixed to the body of the device. Pleated flexible connectors attached to the supply and exhaust ductwork allows freedom for the tilting movement. Face and by-pass dampers may also be used to achieve capacity control. The capacity of the device will vary according to the numbers of coil rows, the fin spacing and the air velocity across the coil. Supply and Exhaust ductwork cannot be separated. Module sizes range from 50 l/s to 36000 l/s. Main Features: Materials of construction: • Tube Material – Copper or Aluminium • Fin Material – Aluminium Performance: • Effectiveness: 45 – 65% (Sensible). The effectiveness of the heat exchanger is dependent on the relative direction of airflows in the supply and exhaust ducts. Counterflow gives the best performance. Parallel flow will reduce the efficiency by about one fifth of the quoted counterflow performance. • Resistance to air flow: Approx. 150 – 500 Pa • Manufacturer’s claim there is no risk of cross contamination between air streams, but this is subject to the quality of the construction of the device. • This technology is seldom used in HVAC applications due to the relatively high capital cost. Typical Applications: • General HVAC systems in Commercial or Industrial applications where ‘sensible’ heat recovery is required. • Particularly where plant space is tight and supply and exhaust duct are running side by side. • Used as an alternative to Plate Heat Exchangers on air handling unit installations where plant space availability is an issue. Note: Only a viable alternative on air volume flowrates exceeding 5-6 m3/s. • Used in dehumidification applications, e.g. Swimming Pools
4.1.4
Air-to-Air Plate Heat Exchanger
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Figure 3 – Cross-Flow Air-to-Air Plate Heat Exchanger Plate Heat Exchangers are available in many configurations, materials, sizes and air flow patterns. They are manufactured in modular form, arranged to handle any airflow, effectiveness and pressure drop requirement. They are simple devices with no moving parts. The two air streams (supply and extract) may be directed in Cross-flow or Counter-flow form, however design, construction and cost restrictions usually result in the cross-flow arrangement being favoured. The casing of the unit is compartmented to form narrow passages carrying, alternatively, supply and extract air streams. The construction of the plates permits a large surface area to be packed into a compact space. Heat is transferred by conduction through the separating plates. The plates are normally constructed from aluminium or stainless steel. An anti-corrosion coating may be applied to the to heat transfer plates when the units are installed in a corrosive environment. Alternatively, Polymer plates with anti-corrosion properties are also available. Capacity control of the device is achieved through the use of face and by-pass dampers. Most plate heat exchangers are provided with condensate drains to remove the condensate formed in the exhaust air section as the heat transfer takes place and the relative humidity of the air increases. Freezing of the exhaust air is a potential problem. This can be controlled by preheating the incoming supply air or by-passing the heat exchanger with part of the incoming air. Units are available to handle air volumes in the range of 25 l/s to 50000 l/s and may be built in modular form to suit any requirement. Supply and Exhaust ductwork cannot be separated. Main Features: Materials of construction: • Plate material – Aluminium, Stainless Steel and Polymer. Paper is used in small Commercial plate heat exchange units. • Casing material: Steel
Performance: • Effectiveness: 50 – 80% (Sensible). Plate Heat exchangers can achieved higher efficiencies because of the single stage heat transfer process and are therefore not inhibited by additional secondary losses associated with the run-around coil arrangement.
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• Resistance to air flow: Approx. 150 – 300 Pa at a face velocity of 3 m/s. With differential pressures in excess of 1000Pa, high delta-P plate heat exchangers should be selected to avoid plate deformation. Typical Applications: • Central Plant Heat Recovery from exhaust air for space heating of Offices, Warehouses and Factories. • Local ceiling mounted heat recovery units for small Commercial applications.
4.1.5
Rotary Air-to-Air Heat Exchanger
Figure 3 – Rotary Air-to-Air Heat Exchanger A rotary air-to-air heat exchanger has a revolving wheel filled with an air-permeable medium having a large internal surface area. The wheel is mounted in a supporting structure, and the motor driven at up to approximately 20 revolutions per minute. Adjacent supply and exhaust air streams each flow through half of the wheel in a counter-flow pattern. The heat transfer medium and the form of heat transfer surface varies between manufacturers and can be selected to pick up ‘Sensible’ heat only or ‘Total’ heat (sensible and latent heat). Sensible heat is absorbed at the warm room exhaust section (exhaust air) and released into the cold outside air section (supply air). Because rotary exchangers are compact, counter-flow devices with small flow passages they can achieve high heat transfer effectiveness. Air contamination, exhaust air temperature and supply air properties influence the selection of materials for the casing, rotor structure and heat transfer medium for the heat exchanger. The exchanger media are fabricated from metal, mineral or synthetic materials and provide either random or directionally orientated flow through their structures. Cross-leakage, cross-contamination or mixing between supply and exhaust air streams can occur in all rotary heat exchangers by two mechanisms – Carry-over and Seal Leakage. Carry-over occurs when air from the exhaust air stream is carried into the supply air stream. This happens each time a portion of the matrix passes the seals dividing the supply and exhaust air sections. It can be prevented by installing a purge section on the heat exchanger. The introduction of a purge section can reduce the level of carry over to reasonable limits, e.g. 0.1%, but cannot completely eliminate it. Cross-leakage occurs due to pressure differentials between air streams. Air leakage is driven from the region of high pressure to the region of low pressure. This can be minimised by avoiding large pressure differentials, providing an effective seal, and placing the fans to promote leakage into the exhaust air stream.
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In many HVAC applications carry-over or cross-contamination is not a concern, however in critical applications such as Laboratories, Cleanrooms or Operating theatres stringent control of carry-over is required. Two methods are commonly used to control the operation of a rotary heat exchanger – Supply air by-pass control (outside air) and speed control of the rotating wheel. In supply air by-pass control a by-pass damper controlled by a supply air temperature sensor regulates the volume of outside air allowed to pass through the rotating wheel and therefore controls the temperature of the supply air. The second method regulates the rate of heat recovery by controlling the speed of the rotary wheel using a variable speed drive. Heat recovery increases with wheel speed, but so does carry-over. Wheel speed is ultimately limited by carry-over. Maintenance requirements need to be taken into consideration since the rotary wheel is difficult to clean. Wheels are available in sizes up to 5.5m diameter to handle air quantities in the range of 25 l/s to 35,000 l/s. Main Features: Materials of construction: • Casing material: Aluminium, Steel and Polymer. • Rotor material: Aluminium, Steel and Polymer. • Exchanger Media: Aluminium, Stainless Steel, Copper or Monel Performance: • Effectiveness: 50 - 85% (Sensible) • Resistance to air flow: Approx. 150 Pa at a face velocity of 3 m/s Typical Applications: • General HVAC systems in Commercial or Industrial applications where ‘sensible’ heat recovery is required. This technology (using a suitable media) may also be used for Total heat transfer (sensible and latent). • Not suitable in applications where cross-contamination may be an issue.
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5.0 SPECIFIC NUMBERS 5.1 5.1.1
Heat Recovery Efficiency The heat recovery efficiency or ‘Effectiveness’ of a device is normally defined as follows: Effectiveness = Actual Heat Transfer Maximum possible heat transfer Exit properties of two air streams can be estimated by knowing the flowrates and the effectiveness of the two air streams. The effectiveness of a heat recovery device can be used as a measure of the heat recovered.
5.2
Comparison of Air-to-Air Heat Exchangers Page 15 of 29
5.2.1
Table 5.0 below shows a comparison of the different types of Air-to-Air Heat Exchangers
Table 5.0
Run-around coil
Heat Pipe
50 - 65% Sensible only
45 - 65 % Sensible only
Temperature Range (ºC)
- 45 – 500ºC
- 40 – 40ºC
- 60 – 800ºC
- 55 – 800ºC
Typ. Pressure drop (Pa)
150 – 500 Pa
150 – 500 Pa
150 – 300 Pa
100 – 300 Pa
Typ. Face Velocity (m/s)
1.5 – 3.0 m/s
2.0 – 4.0 m/s
1.0 – 5.0 m/s
2.0 – 5.0 m/s
Air Flow Arrangements
N/A (1)
Counter-flow Parallel-Flow
Counter-flow Cross-Flow
Counter-flow Parallel-Flow
Cross-Leakage (%)
0
0
0 – 5%
1-10%
Modulation Control
Pump speed control or by-pass valve
Tilt angle down to 10% of maximum
By-pass damper
Wheel speed or by-pass damper
50 l/s and up
50 l/s and up
25 l/s and up
25 – 35000 l/s
2 (incl. pipework)
1
4
3
20 – 30 yrs
15 – 20 Yrs
25 – 30 Yrs
15 Yrs
Heat Recovery Effectiveness (%)
Equipment size range (l/s) Relative Capital Cost (2) Plant Life Expectancy (yrs) Advantages
50 - 80 % Sensible only
Rotary Heat Wheel 50 - 85 % Sensible 50 - 85 % Latent
- Flexibility, i.e. Air streams can be separated
- Few moving parts (except tilting mechanism)
- No moving parts
- Relatively high heat transfer efficiency
- No risk of crosscontamination
- High pressure differentials between air streams is possible
- Low Pressure drop, but can be selected for high pressure differentials.
- Low Pressure drop
- Easily cleaned
- Easily cleaned
- Plate material can be selected to suit a wide range of applications
- Compact large sizes
- Standard, well proven coil technology
- Relatively high heat transfer efficiency
- Matrix material can be selected to suit a wide range of applications
-Relatively space efficient
- Extensively used in Residential and Commercial applications
- Suitable for retro-fit to existing ductwork systems.
Disadvantages
Plate Heat Exchanger
- Two stage heat transfer leading to inefficiencies - Predicting performance requires accurate analysis
-Relatively space efficient
- Few suppliers
- Sensible heat transfer only
- Air Streams must be adjacent
- Relatively low heat transfer efficiency.
- Some risk of crosscontamination (depends on construction quality)
-Filtration required to protect coils - Sensible heat transfer only
- Some risk of crosscontamination
- Cross-contamination between air streams
- Air Streams must be adjacent
- Air Streams must be adjacent
- Sensible heat transfer only
- Fan location is important
- Large space required to accommodate the wheel - Regular maintenance is required.
- Sensible heat transfer only
- Large surface area to volume of matrix makes it susceptible to corrosion.
1. N/A = Not Applicable 2. 1 = Most Expensive, 4 = Least Expensive. Cost comparisons are based on systems handling a volume flowrate of 5 m3/s.
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6.0 AT DESIGN OR RETRO-FIT? 6.1
It is generally more cost effective to include for heat recovery at design stage than to consider it post project completion. Early consideration allows the full range of options to be technically and commercially evaluated, and the best solution selected for the particular application. Once the project has been completed the opportunities are limited.
6.2
Of the technical items outlined in section 3.3 the following will dictate the final selection in a retro-fit situation, a. Plant space availability b. Distance between supply and extract air streams c. Quality and condition of the supply and extract air streams. d. Modification requirements to existing services to accommodate new equipment. e. Move-in space provision. f. Impact on the performance of the existing air handling plant.
6.3 6.3.1
6.4 6.4.1
6.5 6.5.1
6.6 6.6.1
6.7 6.7.1
6.8 6.8.1
Plant space availability Run-around coils and heat pipe heat exchangers will generally occupy less plant space than Plate Heat Exchangers or Heat Wheels. Distance between supply and extract air streams In order to accommodate Rotary Wheels, Plate heat Exchangers or Heat pipes supply and extract air streams need to be in close proximity. With run-around coils supply and extract air streams can be separated by a considerable distance. Quality and condition of the supply and extract air streams Rotary Wheels and Plate Heat Exchangers are susceptible to cross-contamination due to crossleakage. Heat Pipes and run-around coils present no issues with cross-contamination. Modification requirements to existing services to accommodate new equipment The installation of Heat Pipes, Rotary Wheels and Plate Heat Exchangers require that the connecting ductwork services are in close proximity. The extent of modifications required to existing services to accommodate the new equipment will depend on the routing of the services and the space available to accommodate the additional equipment. Run-around coils can generally be accommodated with significantly less disruption to existing ductwork services. Move-in space provision The available move-in space requirements for new equipment will impact on the final selection. Rotary Wheels, Plate Heat Exchangers and Heat Pipe assemblies are relatively large when compared to a run-around coil assembly, which can be broken down into it’s component parts. Impact on the performance of the existing air handling plant All new equipment will impact on the performance of existing plant to which they are connected. Careful consideration is needed to ensure the pressure drop at the new equipment aligns with the available static pressure at the existing plant.
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7.0 WORKED EXAMPLE 7.1 7.1.1
Below is a worked example based on the use of a Run-around coil Heat Recovery device. The resultant heat energy savings, running cost savings and payback period are based on the following input data, 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
7.1.2
Weather Data – Dublin 1966 to 1995 (Refer to Appendix 1 to this document). The range of temperatures is taken from -5°C to 21. 24 hour operation of the air handling system and heat recovery device. Boiler efficiency: 90% Fan efficiency: 80% The boilers are gas fired. The price of Natural Gas is 4.0 Cent per kWh. The price of Electricity is 10.0 Cent per kWh. The extract air temperature is 22°C. The run-around coil system ‘Effectiveness’ is 49%. The supply and extract air volumes flowrates are balanced at 5.0 m3/s respectively. The supply air heat recovery coil has an airside pressure drop of 104 Pa. The exhaust air heat recovery coil has an airside pressure drop of 138 Pa. The volume flowrate of the water/glycol circuit is 5.3 kg/s. The waterside pressure drop in the water/glycol circuit is 100 kPa. The efficiency of the circulating pump in the water/glycol circuit is 65%. The Total Heat Recovery savings are based on an external temperature limit of 14°C.
Appendix 2 to this document includes equipment selections for alternative Heat Recovery devices using the above design parameters.
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Run-around Coil Installation – Worked Example - Calculation Results
Run-around Coil Installation – Simple Payback Period
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8.0 REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
Eurovent 10/1 (1986) – ‘Heat Recovery Devices – Specifications, Terminology, Classification and Functional Characteristics’. Eurovent 10/3 (1989)– ‘Heat Recovery / Energy Conservation – Some Typical Methods’. Besant R.W., C.J. Simonson, Wei Shang – ‘Design for Air-to-Air Heat and Moisture Exchange in HVAC Applications’. ASHRAE Handbook (2007) - ‘ Heating, Ventilating and Air-Conditioning – Applications’. BSRIA TN 11/86 (1986) – ‘Selection of Air-to-Air Heat Recovery Systems’. Article – Bureau of Energy Efficiency ‘ Waste Heat Recovery’. BSEN 13053 (2001) ‘Ventilation for buildings- Air Handling Units – Ratings and performance for units, components and sections’. BG2/2009 – ‘Illustrated Guide to Ventilation’, BSRIA. CIBSE Guide B – ‘Heating, ventilating, air conditioning and refrigeration’ Chartered Institution of Building Services Engineers, London.
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APPENDIX 1 DUBLIN 30 YEAR WEATHER DATA – 1965 to 1995
APPENDIX 2
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Alternative Air-to-Air Heat Exchanger selections • • • • • •
Selection Summary Sheet Air-to-Air Run-around Coils – Fresh Air Coil Air-to-Air Run-around Coils – Exhaust Air Coil Plate Heat Exchanger Rotary Heat Exchanger – Heat Recovery Wheel Heat Pipe
Selection Summary Sheet
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Air-to-Air Run-around Coils – Fresh Air Coil
Page 24 of 29
Air-to-Air Run-around Coils – Exhaust Air Coil
Page 25 of 29
Air-to-Air Plate Heat Exchanger
Page 26 of 29
Air-to-Air Heat Recovery Wheel
Page 27 of 29
Heat Pipe
Page 28 of 29
Page 29 of 29
