Product and air temperatures of the MT and LT cabinets were used as a measure of establishing whether the CO2 refrigeration system had reached steady state condition.
When the product temperatures of the MT and LT cabinet were relatively stable in the range M1 and L1 temperature classifications (as specified in Section 5.2.1) respectively, the MT and LT systems were considered to have reached steady state conditions. These conditions were used for the analysis of the CO2 refrigeration cycle performance as described in the previous sections.
Figure 5.9 Variation of the MT product temperatures with time (Investigated at circulation ratio of 1.3 and evaporating temperatures of -7 oC) Investigated at circulation ratio of 1.3, condensing and evaporating temperatures of -7 oC and -32 oC; df = defrost
Steady state conditions
MT and LT systems reaching the set points Defrost
effect
Defrost effect df Reaching MT set point df
COPLT
COPoverall
COPMT
Time (minutes)
COPMT COPLT and COPoverall
Figure 5.9 shows product temperature variation of the MT cabinet which includes cooling down and steady state period. These results were obtained with a circulation ratio CR = 1.3 and evaporating temperature Tevap,MT = -7 oC. From the figure, it can be seen that the product temperatures can reach the M1 temperature range in 15 hours. The figure also shows that the product temperatures increase during the defrost cycle and when the cabinet reaches the set point.
Figure 5.10 Positions of the product temperature measurements for the MT cabinet
The positions of product (M-package) temperature measurements for the MT cabinet are given in Figure 5.10. Mean, maximum and minimum temperatures of the M-packages at all measured positions during the steady state period of 12 hours are presented in Table 5.2. The product temperatures are grouped according to their positions on the shelves: top, middle and bottom. It can be seen that all measured product temperatures are within M1 classification ranging from +5 oC to -1 oC. The products on the middle shelf have the lowest arithmetic mean temperature (AMT) and the bottom shelf has the highest AMT. The maximum MT product temperature was found to be 5.3°C. This product is located on the bottom shelf at position Bot-10 (Figure 5.10). The overall AMT of the MT products was found to be 1.9 oC (Table 5.2).
Cross flow in the test chamber was from right to left direction; Bot = bottom; Mid = middle 1
Table 5.2 Mean, maximum and minimum temperatures of the MT products Measured
positions Max. Min. Mean Measured
positions Max. Min. Mean Measured
positions Max. Min. Mean Top-01 3.8 1.8 2.9 Mid-01 2.1 0.5 1.2 Bot-01 2.1 1.0 1.6 Top-02 3.7 1.8 2.8 Mid-02 2.3 0.8 1.5 Bot-02 2.8 1.8 2.3 Top-03 0.1 -1.4 -0.5 Mid-03 -0.4 -1.3 -0.9 Bot-03 -0.2 -1.2 -0.7 Top-04 1.0 -1.2 -0.1 Mid-04 0.0 -1.2 -0.5 Bot-04 2.0 0.6 1.3 Top-05 4.7 2.7 3.9 Mid-05 4.1 2.7 3.5 Bot-05 2.9 1.6 2.3 Top-06 4.4 2.8 3.6 Mid-06 3.4 2.1 2.8 Bot-06 4.8 3.4 4.2 Top-07 0.3 -0.8 0.1 Mid-07 0.5 -1.1 -0.2 Bot-07 1.8 0.7 1.3 Top-08 0.8 -0.8 0.0 Mid-08 0.6 -0.8 -0.1 Bot-08 4.0 2.7 3.4 Top-09 5.1 3.6 4.5 Mid-09 4.1 2.9 3.6 Bot-09 4.5 3.4 4.0 Top-10 4.4 3.1 3.8 Mid-10 2.8 1.6 2.2 Bot-10 5.3 4.0 4.7 Top-11 5.0 0.4 2.4 Mid-11 1.6 0.1 0.8 Bot-11 2.5 1.2 1.9 Top-12 1.4 0.5 1.0 Mid-12 1.0 -0.2 0.4 Bot-12 4.0 2.8 3.3 AMT of the top shelf 2.0 AMT of the middle shelf 1.2 AMT of the bottom shelf 2.5
AMT overall of the MT products 1.9
AMT = average mean temperature; Bot = bottom shelf; Mid = middle shelf; Top = top shelf; Max. = maximum temperature; Min. = minimum temperature. Measured positions of the MT products refer to Figure 5.10
Figure 5.11 Variation of air temperatures and RHs of the MT cabinet
Variations for air temperature entering the MT evaporator coil (air-on) and leaving the coil (air-off) are shown in Figure 5.11. The temperatures are average of three point measurements measured at the right, centre and left hand side of the air passage of the MT cabinet before and after the evaporator coil. It can be seen that the air temperatures vary periodically during the test due to the defrost cycle. The temperature difference
Frost accumulation showed effect to the ∆Tair
in the last 2 hours of defrost cycles
between the air-on and air-off is approximately 10 oC after start up and just after defrost.
The temperature difference starts to increase after 4 hours from defrost and reaches more than 12 oC just before the next defrost cycle.
Figure 5.11 also shows the variation of relative humidity (RH) at evaporator coil air-on and air-off measured at the centre section of the MT cabinet. It can be seen that the relative humidity of the air-on and off tends to equalise during the defrost cycle when refrigerant is not flowing through the coil. During the cooling cycle the difference between the RH value on and off the coil tends to increase with frost accumulation on the coil and reaches maximum just before defrost.
Figure 5.12 Variation of the LT product temperatures with time
(Investigated at condensing and evaporating temperatures of -7 oC and -32 oC respectively)
Figure 5.12 shows the variation of the product temperatures of the LT cabinet. The results were obtained at evaporating and condensing temperatures -32 oC and -7 oC respectively and with the doors of the cabinet closed. The temperatures reached the L1 temperature classification (below -15 oC) in 15 hours. At a stable condition, the difference between the maximum and minimum product temperatures was about 5 oC.
Figure 5.12 also shows the super-cooling behaviour of the M-packages when their temperatures first dropped below their freezing point of -1 oC.
Positions of the measured product (M-packages) for the LT cabinet are shown in Figure 5.13. Detailed results of the LT product temperature measurements which include
Steady state conditions
Super-cooling temperature of the
M-packages
AMT overall = -20.7 oC
Highest temperature of the warmest M-package = -15.7 oC
Lowest temperature of the warmest M-package = -20.1 oC
maximum, minimum and mean temperatures are presented in Table 5.3. It can be seen that all of the measured temperatures were in the range of L1 classification. The highest product temperature was on the top shelf at position Top-11.
Figure 5.13 Positions of the product temperature measurements for the LT cabinet
Table 5.3 Mean, maximum and minimum temperatures of the LT products Measured
positions Max. Min. Mean Measured
positions Max. Min. Mean Measured
positions Max. Min. Mean Top-01 -20.3 -22.6 -21.6 Mid-01 -19.2 -22.9 -20.9 Bot-01 -18.8 -22.0 -20.8
AMT overall of the LT products -20.7
AMT = average mean temperature; Bot = bottom shelf; Mid = middle shelf; Top = top shelf; Max. = maximum temperature; Min. = minimum temperature.
Measured positions of the LT products refer to Figure 5.13.
(a) Left section
Cross flow in the test chamber was from right to left direction (b) Centre section
Figure 5.14 shows the temperature variation of the air-on and air-off the LT evaporator coil. These temperatures were the averages of three measurements on the right, centre and left sections of the LT cabinet before and after the evaporator coil. The fluctuations in the temperatures that can be observed are due to the closure of the expansion valve which switched the flow of refrigerant off when the cabinet set point temperature was reached. At steady state conditions the difference in temperature between on and air-off the coil was around 8 oC. The figure also presents the variation of the relative humidity on and off the coil measured at the centre section of the LT cabinet. During steady state conditions, the difference between the air-on and air-off relative humidity was approximately 25%.
Figure 5.14 Variation of air temperatures and RHs of the LT cabinet