Temperature Distribution of Rotary Heat Recovery Units

Transcription

1 Temperature Distribution of Rotary Heat Recovery Units Bjørn R. Sørensen, dr.ing. Narvik University College, P.O.Box 385, N-8505 Narvik, Norway Corresponding SUMMARY This paper deals with measurements of temperature distribution of rotary heat recovery units. A special grid of thermocouples has been made to measure air temperature in equally distributed sampling points on each side of a heat recovery wheel. Air flow rates and the amount of recirculation air (leakage) were measured using a tracer gas. The results show that the temperature profile is generally dependent on both the rotational speed of the wheel and the flow rate. Measurements also show that the placement of temperature sensors in the duct can be significant, since wrong placements may lead to both faulty or non-optimized control of the units, and wrong readings of the temperature efficiency. Propitious sensor placements have been proposed. INTRODUCTION There are many types of air-to-air heat recovery units (HRU) which are in use in ventilation systems (see for instance Schild [1]). Rotary heat recovery exchangers are widely used, and the units are known for their high efficiency and trouble-free operation. Temperature efficiencies above 80% are not uncommon. For balanced ventilation systems, the recovered heat power can be at the same rate. Unfortunately these units are also infamous for transmission of polluted air from exhaust to supply. Due to the rotation of the device, there will always be a direct and indirect connection between exhaust and fresh supply air. Crosscontamination through rotary heat recovery units comes from two mechanisms; (1) Leakage, (2) Carryover. The former is dependent on the differential static pressure across the wheel, and the placement of supply and exhaust fans may affect its value significantly. The latter (carryover) occurs as a result of air being entrained within the wheel volume. Contaminated air is then carried into the other, clean air stream. The degree of carryover air is strongly dependent on whether the unit has a purge section or not. Rotor speed is normally relatively low and in a range of 3-15 rpm. The cross sectional temperature distribution on the supply or discharge side of a rotary HRU is in many cases non-uniform. This means that placement of temperature sensors for control or for calculation of energy transfer and efficiencies is crucial. SITE DESCRIPTION AND MEASUREMENT SETUP Flow and temperature measurements where performed in a laboratory test ventilation system (the heat recovery AHU part of the system is shown in figure 1). This is a ABB EC2000 unit with a non-hygroscopic rotary wheel. Flow rates where measured with a tracer gas. A tracer gas (SF 6 ) was released into the air, and concentrations upstream and downstream of the dosing points determined the flow rate and cross-contamination of the heat recovery unit (according to figure 2).

2 Figure 1. The AHU subsystem containing the rotary heat recovery unit Figure 2. Flow measurement setup and tracer gas sampling points, Eian/Sørensen [2] Figure 3. Temperature sampling points

3 The temperature measurement system (refer to figure 3) consisted of: Three grids of thermocouples on the intake and supply side (grid T1, T2 and T3), each containing 6 sample points (figure 3 and 4) Three grids of thermocouples on the exhaust and discharge side (grid A1, A2 and A3), each containing 6 sample points Temperature in the ducts T0, T4, A0 and A4. A NI1000 mean temperature sensor on the exhaust side (this sensor was not used in this study) Figure 4. Picture of a thermocouple grid in front of the rotary heat recovery unit. RESULTS AND DISCUSSION Flow rates Figure 5 shows the flow rates that were measured for different speeds of the fan and heat recovery unit. Figure 6 shows the percentage return air being recirculated to the supply side. Flow rate (m3/h) Flow rate - exhaust 100% rotary speed 50% rotary speed 0% rotary speed Cross contamination (%) 100 % 90 % 80 % 70 % 60 % 50 % 40 % 30 % 20 % 100% rotary speed 50% rotary speed 0% rotary speed % 0 0 % 20 % 40 % 60 % 80 % 100 % Fan speed (% of max) 0 % 0 % 20 % 40 % 60 % 80 % 100 % Fan speed (% of max) Figure 5. Flow rates measured in the exhaust Figure 6. Cross contamination (%)

4 A large amount of recirculated return air was measured. The large amount of return air was mainly a result of the rotary HRU having no purge sector and an unfavorable placement of the fans in the system. This resulted in a low static pressure on the intake side of the HRU and a relative high pressure on the exhaust side, forcing air to cross to the supply side. The latter caused a 12-15% air recirculation. It was also noticed that the amount of recirculation increased significantly with increased speed of the HRU wheel. This indicates that carryover air stands for a large amount of total recirculation. Furthermore, at low fan speeds, the relative amount of recirculated air increased significantly. While the fans were running at only 20% of maximum speed, and the HRU was at full speed, there were almost no fresh outdoor air on the supply side (90% recirculation). Thus this confirms the importance of obtaining proper pressure conditions around the rotating wheel. The results also suggest that for VAV systems running on low fan speeds, the amount of return air can become very high and thus unacceptable. Measurements with the HRU not rotating show an almost constant amount of recirculated air (independent of fan speed). Hence, the increase of recirculation was mainly caused by carryover air in the rotating wheel. Furthermore, the amount of carryover air from the supply to the exhaust was quite noticeable, as can be seen in figure 5 (since flow rates on the exhaust side varied with the speed of the rotating wheel). Similar measurements were performed on the supply side of the HRU, showing that the ventilation system was very good balanced with respect to supply and exhaust flow rates. Temperature measurements Relative speed (%) 120 % 100 % 80 % 60 % 40 % 20 % 0 % Fan speed HRU w heel speed Time (min) Figure 7: Fan and HRU wheel speeds (% of max) during temperature measurements Two measurements series were performed, each resulting in responses from altered fan speeds (0-100% of max, 20% step) and HRU wheel speeds (0%, 10%, 20%, 40%, 60% and 100% of max), see figure 7. The outdoor temperature was fairly constant around 0 C. Figure 8 shows the temperature measurements as a function of time from grid T2 (supply side, closest to the wheel) for measurement series 2. At a first glance, it is obvious that the temperature varied quite noticeable across the section/grid for a given fan and HRU wheel speed. If looking closer it is possible to see the variations in temperature due to the alternations of the wheel rotary speed. The most conspicuous temperature drops happened when the rotary HRU was shut down to 0% speed. When this occurred, the temperature on the supply side naturally dropped towards the outdoor temperature. In figure 9-12, the results have been arranged so that they correspond to the location of the actual sampling point. The two upper rows of graphs correlate to the grids on the supply side of the HRU wheel, grid T2 and T3, while the two lower rows of graphs correlate to the grids on the discharge side, grid A2 and A3. In addition, the duct temperatures (T4 and A4) are plotted on the supply and discharge graphs respectively. Since these sampling points are located away from the wheel (in the ducts), they are used as references to decide whether a

5 temperature value is too low or too high in the grids. Each graph has been numbered in accordance with the setup given by figure 3. The direction of the HRU is also shown. Figure 9 and 10 show steady state temperature levels (according to the variations in fan speed and wheel speed defined by figure 7) for each sampling point at 20% respectively 100% fan speed, as a function of the relative HRU wheel speed. Likewise, figure 11 and 12 show temperature levels for each sampling point at 20% respectively 100% HRU speed, as a function of the relative fan speed. Temperature ( C) Grid T Time (min) Figure 8. Temperature measurements, grid T2 (supply side, close to wheel). From figure 9 and 10 it can be seen that the HRU speed is of minor importance as long as it is above 20% (in this case, 20% speed corresponds to an absolute speed of 2.25 rpm). Furthermore, while the fans run on low speed (20% of max) the cross sectional stratification bedding is small on the supply side, and the temperature distribution is fairly uniform. As the HRU speed is increased, temperatures on the supply side decrease slightly. Most likely this is caused by the large increase of recirculation air (refer to the flow measurements discussed earlier). However, in a system with an efficient purge sector this tendency will not show. On the discharge side, the distributions are more scattered, with highest values near the center line of the wheel. When the fan speed is increased (figure 10), the cross sectional temperatures on both the supply and discharge sides of the wheel become increasingly scattered, and a cross sectional stratification is revealed. In this situation, the air is warmest near the center line of the wheel. From the next two figures (figure 11 and 12) it can be seen that the fan speed of course affect the temperature levels significantly. While the fans run on low speed (20% of max) the cross sectional stratification bedding is small on the supply side, and the temperature distribution is fairly uniform. There is however a tendency of high temperatures near the center of the unit and low temperature around the edges. On the discharge side, the warmest air is near the edge of the wheel, close to the center line. Table 1 shows a simple summary of the temperature levels from figure 9-12, as a color map of grid T2 and A2. Levels are shown as +/- differences from the duct levels T4 and A4, which are believed to be more representative for the actual air temperature. Table 1. Map of temperatures ( C) close to HRU wheel (supply and discharge side). Fig. 9, fan 20% Fig. 10, fan 100% Fig. 11, HRU 20% Fig 12, HRU 100% Varying HRU Varying HRU Varying fan Varying fan Supply Discharge T2-1 T2-2 T2-3 T2-4 T2-5 T2-6

6 Figure 9. Air temperatures, T2-T4 and A2-A4 (supply and discharge sides) as a function of HRU rotational speed at 20% fan speed (HRU view from intake side). Figure 10. Air temperatures, T2-T4 and A2-A4 (supply and discharge sides) as a function of HRU rotational speed at 100% fan speed (HRU view from intake side).

7 Figure 11. Air temperatures, T2-T4 and A2-A4 (supply and discharge sides) as a function of relative fan speed at 20% HRU rotational speed (HRU view from intake side). Figure 12. Air temperatures, T2-T4 and A2-A4 (supply and discharge sides) as a function of relative fan speed at 100% HRU rotational speed (HRU view from intake side).

8 Placement of temperature sensors in the duct The ideal placement of a temperature sensor used for Supply control or efficiency calculations is away from the wheel so it is not affected by stratified air nor by radiation from the wheel media. However, this is rarely convenient or possible, so the compromise must be to find a spot near the wheel (on all sides) that approximately resembles the actual air temperature as if it was measured away and shielded from the device. From table 1, an assessment of sensor placement is possible. For this actual HRU wheel Discharge it seems most appropriate to place the supply side and Figure 13. Placement of sensors discharge side sensors as shown by the yellow spots in figure 13. It should also be mentioned that the temperatures on the exhaust and intake side of the wheel were fairly uniform, but small variations can be expected due to radiation from the wheel and in cases with leakage. A similar assessment may be required here. Possible sources of error Measurements should be carried out on several different heat recovery units. Also, the number of repeated measurements series are low, and it is thus not possible to establish a basis for categorical conclusions. Nevertheless, the results serve to pinpoint typical tendencies. CONCLUSIONS The flow rate measurements results suggest that for ventilation systems running on low fan speeds (VAV), the amount of recirculation of air from the exhaust to the supply can, due to carryover, become very high and thus unacceptable. The rotational speed of the heat recovery unit has shown to be of minor importance as long as the wheel is rotating. For this particular case, above 20% of maximum speed. The ideal placement of a temperature sensor used for control or efficiency calculations is away from the wheel so it is not affected by cross-sectional stratification bedding, nor by radiation from the wheel media. For the particular HRU in question, measurement results suggest that the sensors on the supply and discharge side should be placed approximately in the middle of the AHU duct, or displaced to the sides. FURTHER WORK This work represents the first report in a project on air-to-air heat recovery devices carried out at Narvik University College. Future work will address temperature and energy efficiency, transient conditions, control, modelling and simulation. REFERENCES 1. Schild P G Air-to-Air Heat Recovery in Ventilation Systems. AIVC Ventilation Information Paper no Eian P. K., Sørensen B. R. 1996, Bestemmelse av ventilasjonens effektivitet med sporgassanalyse. Report (in Norwegian), Narvik University College, ISBN