REPORT. Condensing air heaters. Technology evaluation. Project report Maj 2013

Transcription

1 Condensing air heaters Technology evaluation Project report Maj 2013 REPORT Danish Gas Technology Centre Dr. Neergaards Vej 5B DK-2970 Hørsholm Tlf Fax

2 Condensing air heaters Technology evaluation Mikael Näslund Danish Gas Technology Centre Hørsholm 2013

3 Title : Condensing air heaters Report Category : Project Report Author : Mikael Näslund Date of issue : Copyright : Danish Gas Technology Centre File Number : ; h:\737\72 kondenserende luftvarme\rapport\luftvarmerapport_final.docx Project Name : Kondenserende luftvarme ISBN :

4 DGC-report 1 Table of Contents Page Summary Introduction Burner Heat exchanger Flue system Laboratory test of one product: Robur G Test procedure Appliance performance Other condensing air heaters on the market Installation aspects Air and flue systems Economy Conclusions References Appendices Appendix A: Update of DGC guideline 45 and Danish fuel gas code, Gasreglementet

5 DGC-report 2 Summary Condensing technology is today the state of the art for single-family house gas heating in Scandinavia. Heating of larger industrial premises and warehouses are still dominated by non-condensing technologies. Condensing air heaters are available on the market but have not yet found widespread use in Denmark. This report describes and evaluates the condensing air heating technology, and test results are reported. The energy saving using a condensing air heater is approximately 15% compared to a modern non-condensing air heater and approximately 20% if an old air heater is replaced. A condensing air heater was laboratory tested. The efficiency at nominal load was measured to 97% which is in accordance with manufacturer data. At minimum load the efficiency was measured to 102.9% compared to 105.7% claimed by the manufacturer. The annual efficiency is estimated to 100%. The emissions were low with a NO x level of 22 mg/mj or lower depending on burner load. The CO levels were almost zero, with the exception of a moderate peak (100 ppm) directly after the burner ignition. The simple pay-back time if the condensing option is chosen instead of a modern non-condensing air heater was calculated to be between 0 and 5 years. This project was funded by the the Danish gas companies' Technical Committee on Gas Utilisation and Installations (FAU GI). Quality assurance at DGC was made by Jan de Wit. Thanks to Danheat for making a Robur air heater available for testing. Thanks also to Stefano Caverzaschi at Robur in Italy for technical comments regarding the Robur air heater.

6 DGC-report 3 1 Introduction Condensing technology is today the state of the art for single-family house gas heating in Scandinavia. Heating of larger industrial premises and warehouses and tool shops are still dominated by non-condensing technologies. Condensing air heaters are available on the market, but have not yet found widespread use in Denmark. Both independently operating unit heaters and central air heating systems are available in high-efficiency condensing designs. The manufacturers claim % steady-state efficiency for condensing units and 90% steady-state efficiency for new non-condensing appliances. This means energy saving of approximately 15% at nominal load using a modern condensing unit instead of a modern non-condensing unit alone. Retrofitting old heater technology with condensing technology increases the energy savings even more. The old Ambirad Centurion air heater with atmospheric burner and open flue has an efficiency of 85% according to Ambirad product description. Using a condensing air heater will then reduce the fuel consumption by approximately 20% if an old air heater is replaced. The main differences between non-condensing and condensing designs are in the heat exchanger, the burner and the flue system, i.e. all important appliance parts. General images of a non-condensing and a condensing unit are shown in Figure 1. The top images show the Lennox LF24 with approximately 90% efficiency. The burner is of a forced draught type and the heat exchanger is a simple tubular heat exchanger where the flue gases flow on the inside. The heat exchanger surface is smooth without any heat transfer enhancing fins etc. The bottom image shows an Ambirad UESA condensing air heater. The heater is marketed under at least three brands: Ambirad, Benson and Reznor. It has a single-stage burner firing horisontally towards the heat exchanger inlet. This model, as well as other condensing air heaters, has a heat exchanger in two clearly visible parts. It resembles some early condensing boilers for single-family houses which also had a second condensing heat exchanger.

7 DGC-report 4 Lennox LF24 Ambirad UESA Figure 1 A traditional non-condensing (Lennox LF24) and a condensing (Ambirad UESA) unit heater The manufacturers of new highly efficient non-condensing and condensing air heaters often mention the following characteristics for the products: High efficiency. Low temperature rise for the air. This means better warm air distribution, reduced stratification and increased comfort. Modulating premix burners. Room sealed flue solutions. 1.1 Burner Burners in condensing air heaters are often modulating premix burners. They offer a constant excess air ratio in the modulation range. The emissions of CO and NO x are also low.

8 DGC-report Heat exchanger The heat exchanger design is also responsible for the temperature and flow profile at the unit air exit. The images in Figure 2 from Ambirad and Robur brochures show the warm air temperature distribution in the heated room. The top left images show the distribution from an old air heater compared to the distribution from a new design shown in the lower left image. The image to the right shows a thermography picture from a Robur air heater. The flow and temperature field is similar to the more general flow field presented in the Ambirad images. This kind of improved temperature distribution in the room could also lead to a reduced heating demand. Figure 2 Illustrations from Ambirad and Robur describing improved warm air distribution with new air heater designs 1.3 Flue system The condensing unit air heaters are all modern regarding the air and flue systems. Normally, room sealed, closed systems are used for stationary installations. However, open flue solutions are often mentioned as an option. Modern non-condensing air heaters are also delivered with room sealed flue systems. Figure 3 shows examples of flue solutions from the manufacturer Mark.

9 DGC-report 6 Figure 3 Examples of flue solutions for modern unit air heaters (Source: Mark) The acceptable flue system lengths for the Robur G30 are taken as an example of appliance specific lengths. Table 1 Flue system lengths specified for Robur G30 condensing air heater Category Description Limit C 13 C 33 B 23 C 53 Room sealed, horizontal flue terminal. The fan is located upstream of the burner/heat exchanger. Room sealed, vertical terminal. The fan is located upstream of the burner/heat exchanger. Open combustion, no draught diverter. Vertical flue system The fan is located upstream of the burner/heat exchanger. Room sealed split system. The fan is located upstream of the burner/heat exchanger. Air: 8 20 m Flue gases: 8 20m Air: m Flue gases: 20 30m Flue gases: 17 m ( =80 mm) 30 m ( =110 mm) Air: 1 m Flue gases: 13 m ( =80 mm) 30 m ( =110 mm)

10 DGC-report 7 2 Laboratory test of one product: Robur G 30 An air heater was tested in the DGC laboratory to verify the manufacturer's claimed efficiency and to investigate the overall characteristics. Robur G 30 unit heater was chosen and the Danish importer Danheat made a unit available for testing. The tests comprised: Steady-state efficiency Emissions Figure 4 shows four external images of the Robur G30 unit and a view of the heat exchanger. The top images show the heater as installed while the two bottom images show the heat exchanger when the front louver and the rear air fan are removed for better visibility.

11 DGC-report 8 Figure 4 Views of Robur G30 air heater The premix burner is mounted in a large cylinder. The burner modulates between 13 and 29 kw with constant excess air ratio. On top of the cylinder are two almost vertical heat exchangers with fins on both the flue gas and the air sides (lower left image). The material is an aluminium alloy. The last part of the heat exchanger, where the condensation occurs, begins at a box on top of the two vertical tubes. The flue gases flow inside four corrugated and flexible stainless steel tubes directed towards the air inlet of the heater

12 DGC-report 9 (lower right image). At the bottom, close to the air inlet, these tubes are connected to a box where condensate also is collected and drained. The flue gases leave the air heater through the outlet at the top left corner of the appliance. The air flow through the heater is managed by an axial fan. The capacity is m 3 /h within the burner modulating range. The temperature increase is stated to 16 C at minimum fan speed and 32 C at maximum speed. All data according to Robur data sheets. The Robur G series condensing air heaters are available in 4 sizes with nominal capacities between 30 kw and 90 kw. 2.1 Test procedure Test conditions for condensing air heaters are specified in the European test standards [1] and [2]. DGC has not previously tested air heaters and the test procedure did not fully comply with the test standards. The test set up is shown in Figure 5. The air heater is placed between two wooden shields which direct the flow to the ventilation system. The warm air enters into the 400 mm hose and is finally evacuated through the laboratory ventilation system. This is necessary to keep the laboratory temperature within reasonable limits. The image to the right shows the air intake and the flue system. Laboratory air is taken directly into the air heater. The flue system is a split option where combustion air is taken from the laboratory. The combustion air temperature is then equal to the laboratory temperature. Figure 5 Robur G30 air heater test set up

13 DGC-report 10 The air heater was connected to gas and electricity. The burner was checked and not adjusted to Danish natural gas. The current installation procedures for premixed burners in Denmark suggest no burner adjustment to current gas quality. The appliance should keep the factory adjustment to pure methane, G20. The steady-state tests were done during 20 minutes after a stabilization period. Gas input and air temperatures were measured. A standard flue gas analysis including O 2, CO 2, NO x and CO measurements were also made. 2.2 Appliance performance The efficiency was measured in the four operating points possible to set in the control box. These operating points are shown in Table 2. Table 2 Operating conditions in testing of Robur G30 air heater Operating point Burner input Fan speed 1 30 kw (Max) Max 2 22 kw High 3 22 kw Low 4 16 kw (Min) Min The laboratory air temperature was C, which is within the test standard limits. The O 2 content in the flue gases varied from 4.5% at maximum gas input to 5% at minimum gas input. The measured flue gas temperature and efficiency are shown in Figure 6. It is clearly seen that the flue gas temperature in general is never below the dew point. The condensate is formed due to cold heat exchanger wall temperatures. There is almost no condensate found at maximum gas input while the condensate formation at minimum load was measured to 0.5 l/h. At maximum burner input the flue gas temperature is in the C range while it is C at minimum fuel input. The flue gas temperatures are above the dew point in every operating condition. Condensation takes place due to wall temperatures below the dew point.

14 Temperature (C), Efficiency (%) DGC-report 11 Robur G30 air heater - flue gas temperature and efficiency Flue gas temperature Efficiency Gas input (kw) Figure 6 Robur G30 air heater laboratory performance data Robur claims an efficiency of 105.7% at minimum burner input. This was not measured in the DGC laboratory tests. The maximum efficiency in the DGC tests was 102.9%. The tests were repeated, but without any change in the maximum efficiency. The efficiency was calculated according to European standards [1], [2]. The efficiency for condensing units is the sum of two parts. The first efficiency part is based on temperature and excess air ratio only. The second efficiency part is the energy from the collected condensate volume during the test. The spread sheet for the efficiency calculation used by Robur was also used for comparison. No differences were found. One explanation may be the gas quality. The Robur data is obtained with pure methane. The DGC laboratory measurements were done with Danish natural gas with a higher Wobbe number. This means that the burner input is higher and the inner wall temperature in the condensing part of the heat exchanger may be slightly higher. A higher wall temperature will reduce the condensate formation and the efficiency gain from the condensation. The flue gas temperature is the same as in the data from Robur which indicates that it is the condensate formation that affects the results. Robur has suggested other possible sources to the difference in efficiency. The most likely seems to be the air temperature. Air temperatures below 20 C would increase the condensate formation. However, the air tempera-

15 Temperature difference T_flue - T_room (C) DGC-report 12 ture during the tests should be 20±5 C as stated in the test standard /2/. The standard seems to allow a fairly wide temperature range for a condensing appliance. Another suggested explanation is that some formed condensate did not drain properly and remained in the heat exchanger but the repeated tests showed almost identical condensate formation rates. The combustion is clean with low NO x emissions and very low CO emissions. The emissions of NO x and CO are shown in Table 3. The CO emission peaks shortly after the burner starts with a maximum of approximately 100 ppm. The level rapidly decreases to <1 ppm within 2 3 minutes. This is close to the instrument detection limit. Table 3 Robur G3 air heater emissions Load (kw) NOx (ppm 0% O 2) NO x (mg/mj) CO (ppm) CO (mg/mj) The difference between the room air temperature and the flue gas temperature is shown in Figure 7. The inlet temperature equalled the laboratory air temperature and varied between 22 and 25 C during and between the tests. 70 Robur G30 air heater temperature difference Gas input (kw) Figure 7 Robur G30 air heater temperature laboratory data Finally, the electric power demand for operation was measured. For the operating settings the power demand was measured as shown in Figure 8. The

16 Electrical input (W) DGC-report 13 main consumption is caused by the air fan. The two points at approximately 20 kw gas input is explained by two different fan settings. In general, the electricity consumption is approximately 1% of the gas input. 400 Robur G30 air heater power demand G30 gas input (kw) Figure 8 Robur G30 air heater electric power demand

17 DGC-report 14 3 Other condensing air heaters on the market Table 4 shows the manufacturer data for a number of unit air heaters. A condensing air heater is compared to a non-condensing air heater with similar output. The most efficient non-condensing appliance was chosen if the manufacturer offers several models. The non-condensing designs were chosen to be as close in fuel input as possible to the condensing unit from the same manufacturer, and the data were collected from the websites 1. The models from each manufacturer are not always comparable since the designs may be significantly different. Table 4 Manufacturer data for some condensing and non-condensing unit air heaters (market status early 2013) Model Robur G30 Robur F131 Ambirad UESA 35 Ambirad UDSA 35 Mark GS+ a Cond. Y N Y N Y N Mark GSE Input (kw) Efficiency max (%) Efficiency min (%) Noise (dba) 47/59 43/55 45/52 45/ Air flow (m 3 /h) Air throw (m) Temp. rise (K) Elec cons. (W) Weight (kg) a Mark GS+ is also marketed as Winterwarm HR The condensing air heater efficiencies all increase at part-load operation. The nominal load efficiency is between 95% and 97% for all models. The efficiency at minimum gas input is in the % range. The Ambirad figure is probably the minimum load efficiency. No details are given on the website. The tests of the Robur G30 clearly show the differences in operating conditions at nominal and part load. The differences between the models seem not to be larger than between condensing gas boilers

18 DGC-report 15 The efficiencies for the non-condensing air heaters sold in 2013 are quite similar at nominal load, 90 92%. Data for part-load operation are normally not available. The noise levels in the table are either given as the noise in free field or as the noise levels in free field, and in a typical installation. There is no indication that a condensing air heater is much different than a non-condensing heater with respect to the noise level. Larger differences between the air heaters seem to exist in the air flow and throw lengths. However, the definition of throw length is different. Robur defines it as the distance where the air velocity exceeds 1 m/s, while Ambirad defines it as the distance where the air velocity exceeds 0.35 m/s. The differences seen in the table are probably not as significant if the same definition is used. The electricity consumption seems not automatically to be higher in condensing air heaters despite the potentially higher pressure drop across the heat exchanger air side. In most cases the maximum electricity consumption is 1 1.5% of the gas input.

19 DGC-report 16 4 Installation aspects The installation of condensing air heaters is not expected to cause any major difficulties compared to non-condensing air heaters. The condensate flow is limited and the weight is not always higher for condensing air heaters. The weight difference between condensing and non-condensing models indirectly reflects the size and complexity of the condensing heat exchanger. It seems possible to design both a light and efficient condensing heat exchanger. A low weight appliance has also some benefits in ease of installation and the working environment. 4.1 Air and flue systems As previously described the possible air and flue systems are similar to those of condensing boilers for single-family houses in terms of CE marking, materials etc. A slightly larger space may be required if old noncondensing air heaters are retrofitted with new condensing models. It is assumed that the same Danish rules are applicable regarding the appliance and the flue system, i.e. the appliance and the flue system must be approved as a unit and not put together from different sources. 4.2 Economy In this section a brief economic analysis of condensing gas-fired air heaters will be done. In Table 5 the prices for Robur condensing air heaters (G series) and non-condensing air heaters (F series) in Denmark are shown. Table 5 Prices for Robur air heaters in Denmark (excl. VAT) Type Model Heat output (kw) Burner modulation List price (DKK) Condensing G mod G mod G mod G mod Non-condensing F31 30 single stage F51 48 single stage F60 60 single stage F single stage The prices show that the higher-efficiency condensing boilers are more or less equal in price and the pay-back time for the end-user is almost zero. The prices in the table are for the heater only. Mounting kit and flue system

20 DGC-report 17 cost DKK for each heater, regardless of condensing or noncondensing model. As a comparison, the British list prices [3] for Benson condensing and noncondensing air heater are shown in Table 6. Both options have room sealed flue systems. The condensing model equals the Ambirad condensing air heater in Figure 1. The exchange rate in the beginning of February 2013 is 1 GBP = 8.63 DKK. In the table are also the prices for Winterwarm condensing and non-condensing air heaters shown. The prices are from the Dutch web site and in Euro (1 Euro = 7.45 DKK). Table 6 Price comparison for Benson/Ambirad and Winterwarm condensing and non-condensing air heaters Type Model Heat output List price (kw) Benson/Ambirad Condensing UESA ( ) UESA ( ) UESA ( ) UESA ( ) Non-condensing UDSA ( ) UDSA ( ) UDSA ( ) UDSA ( ) Winterwarm Condensing HR ( ) HR ( ) Non-condensing XR ( ) XR ( ) The simple pay-back time is calculated for installing a condensing air heater instead of a non-condensing air heater. The results are shown in Figure 9. The pay-back time is calculated as a function of the annual heat output for an individual heater and the additional cost for choosing a condensing instead of a modern non-condensing option. The assumptions are: Condensing air heater annual efficiency: 100% Non-condensing air heat annual efficiency: 90% Gas price: 0.68 DKK/kWh

21 Pay-back time (years) DGC-report Pay-back time for condensing air heaters DKK additional price DKK additional price DKK additional price Annual gas consumption for one air heater (kwh) Figure 9 Pay-back times for choosing a condensing instead of a noncondensing air heater No data for the annual heat production of individual air heater has been found. If we assume that a heater with 20 kw capacity is sufficient to cover the annual heating demand of an area of approximately 300 m 2, the energy consumption approximately kwh per year. Since a 30 kw heater is at the lower end of heater size we conclude that pay-back times are to be based on a heat production exceeding kwh per year. The additional cost for a condensing air heater according to Table 6 is DKK The pay-back time is then determined between the blue and red curve in Figure 9. Based on the discussion above the pay-back time for a condensing air heater compared to a new non-condensing air heater is estimated to be between 0 and 5 years. The savings in annual gas cost is briefly illustrated in Table 7. The data shows the economic gain in gas cost when an old air heater is replaced by a new condensing air heater with 100% annual efficiency. An efficiency difference of 15% means that the old air heater has an annual efficiency of 85%. The cost reduction varies linearly between the two annual heating demands.

22 DGC-report 19 Table 7 Gas cost reduction (DKK) when replacing an old air heater with a condensing air heater (100%) Heating demand Efficiency difference (kwh/year) 15% 20%

23 DGC-report 20 5 Conclusions Condensing technology is today the state of the art for single-family house gas heating in Scandinavia. Heating of larger industrial premises, warehouses and tool shops are often done by non-condensing technologies. Condensing air heaters are available on the market but have not yet found widespread use in Denmark. The energy saving using a condensing air heater is approximately 15% compared to a modern non-condensing air heater and approximately 20% if an old air heater is replaced. A condensing air heater was laboratory tested. It has a premix burner and closed air and flue systems. The efficiency at nominal load was measured to 97% which is in accordance with manufacturer data. At minimum load the efficiency was measured to 102.9% compared to 105.7% claimed by the manufacturer. The difference may be explained by the slightly higher burner input due to the Danish natural gas used instead of the pure methane used for the specifications. The annual efficiency was estimated to 100%. The emissions were low with a NO x level of 22 mg/mj or lower depending on burner load. The CO levels were almost zero, with the exception of a moderate peak (100 ppm) directly after the burner ignition. The electricity consumption corresponded roughly to 1% of the gas input. The difference in investment cost for a condensing air heater and a state-ofthe-art non-condensing air heater was used to evaluate the economy and simple pay-back time if the condensing option is chosen. With data from three manufacturers and current Danish gas prices the simple pay-back time was calculated to be between 0 and 5 years.

24 DGC-report 21 6 References [1] DS/EN 1020, "Non-domestic forced convection gas-fired air heaters for space heating not exceeding 300 kw, med forbrændingsluftblæser eller røgsuger, ikke til husholdningsbrug," Dansk Standard, [2] DS/EN 1196, "Domestic and non-domestic gas-fired air heaters - Supplementary requirements for condensing air heaters," Dansk Standard, [3] Ambirad Group, "Price list ," [4] Dansk Gasteknisk Center DGC, "Servicering af gasfyrede luftvarmeanlæg, DGC vejledning 45,"

25 DGC-report 22 Appendix A Update of DGC guideline 45 and Danish fuel gas code, Gasreglementet The DGC guideline 45 deals with air heaters. It clearly needs updating, as well as the Danish fuel gas code Gasreglementet. Figure 10 shows the text regarding air heaters in Gasreglementet. Figure 10 Text from Gasreglementet regarding air heaters Paragraph refers to a Danish standard for oil-fired air heaters. It should be adapted to the current European standards for gas-fired heaters. The DGC guideline 45 for maintenance of gas-fired air heating installations (Servicering af gasfyrede luftvarmeanlæg) [4] issued in 2004 needs a revi-

26 DGC-report 23 sion and update. This report does not consider the details. The general comments on the current guideline are as follows: There is a need for updating and checking the recommendations considering new air heater design since References to documents are no longer available. Control of the recommendations regarding burner adjustments. Since the guideline was issued new recommendations on burner adjustment have been developed. The expected future gas quality variations have changed the adjustment method for premixed burners.