Energy savings in commercial refrigeration equipment: High Pressure control implementation

Transcription

1 Energy savings in commercial refrigeration equipment: High Pressure control implementation August 2011 / White paper by Christophe Borlein AFF and l IIF-IIR member Make the most of your energy

2 Économie d énergie dans les centrales frigorifiques : La haute pression flottante Foreword High Pressure control implementation is fairly easy. However refrigerating systems are not always adapted to this regulation and require in depth checks and sometimes modifications. Knowing the effect of High Pressure control on the efficiency of the installation allows to increase energy saving. Being aware of appearance of a problem allows to take preventive actions rather than stopping the High Pressure control, which is the case on many installations. White paper on Energy Efficiency II

3 High Pressure control possible effects.

4 Économie d énergie dans les centrales frigorifiques : La haute pression flottante Impact of the power of the pilot-controlled valve Thermostatic pilot-control valves either mechanical, or electric and electronic control an overheating on the outlet side of the evaporator. In fact they are modulating valves acting on the flow. When the valve is completely open and overheating is higher than setting, that means that the valve is too small. (power of the evaporator should not be sufficient at this time or Low Pressure will go down until a Low Pressure fault is detected). Note: following calculations are simplified and approximate. Howeverr they allow to follow step by step easily and understand why a multi outlets valve is not absolutely necessary. Tables at the end are given by the manufacturers. Data of these tables are the only ones which shoul be taken as irrefutable. Pilot control valves are more or less governed by hydraulics laws as fluid is present in 2 states. With little approximation we can state that pressure loss varies with the square of flow. p = av ² Danfoss data In other words, if the pressure is reduced by 50 %, the flow accross the valve downd by 30 %. There should thus be a refrigerating power loss. R-22 fluid, evaporating at -12 C (2,3 bar) condensing at 40 C (14,3 bar) down to 20 C (8,1 bar) will have a volume reduction approximately equal to 30 % (negletcting the density variation) Figure 1 : TE Thermostatic Valve 1- Thermostatic Element 2- Input cartridge 3- Valve body 4-Overheating adjustment rod 5- Pressure balance White paper on Energy Efficiency 1

5 Économie d énergie dans les centrales frigorifiques : La haute pression flottante High Pressure Using the R-22 refrigerating cycles as shown figure 2, enthalpy is C, down to C, ) i.e. 19 % power loss. This change, close to 20 % does not allow the use a traditional regular pilot-control valve to make High Pressure control. It is thus necessary to have another means of regulation (electronic, analogic, electric, multi-outlets, ) Pressure [Bar] R22 Ref :R.C.Downing. ASHRAE Transactions Paper No DTU, Department of Energy Engineering s in [kj/(kg K)]. v in [m^3/kg]. T in [ºC] M.J. Skovrup & H.J.H Knudsen v= v= v= v= v= v= v= v= v= v= Enthalpy [kj/kg] v= v= x = s = v= s = s = s = 1.70 s = s= s=1.85 s = s =1.95 s= s = 2.05 s = Figure 2 : R22 Refrigeration Cycles condensing at 40 C and 20 C

6 High Pressure The R-404A, evaporating at -12 C (3,1 bar) condensing at 40 C (17,2 bar) down to 20 C (9,9 bar) with a reduction of volume approximately equal to 31 % (identical to R-22) (see refrigerating cycles figure 3). The enthalpy is C and C. Power loss is of 9%. This variation is acceptable and does not prevent the installation to operate correctly R404A Ref :DuPont SUVA HP62 DTU, Department of Energy Engineering s in [kj/(kg K)]. v in [m^3/kg]. T in [ºC] M.J. Skovrup & H.J.H Knudsen s = Pressure [Bar] s = 1.65 s = 1.70 s=1.75 s = 1.80 s=1.85 s =1.90 s = x = s = Enthalpy [kj/kg] 0.15 Figure 3 : R-404A Refrigeration Cycles condensing at 40 C and 20 C White paper on Energy Efficiency 3

7 High pressure This is illustrated by the selection tables of the pilot-control valves manufacturers (Danfoss): Power capability - 40 C à 10 C range Part number Nb of outlets Pressure loss in the valve Dp bar Evaporation temperature -10 C TX 2/TEX X TX 2/TEX TX 2/TEX TX 2/TEX TX 2/TEX TX 2/TEX à 18 % TX 2/TEX TX 2/TEX TEX TEX TEX TEX R22 Power capability - 40 C à 10 C range R404A/R 507 Part number Nb of outlets Pressure loss in the valve Dp bar Evaporation temperature -10 C TS2/TES X TS2/TES TS2/TES TS2/TES TS2/TES TS2/TES TS2/TES à 9 % TS2/TES TES TES TES TES Power losses with the R-404A can be neglected. White paper on Energy Efficiency 4

8 Économie d énergie dans les centrales frigorifiques : La haute pression flottante Conclusion A refrigerating equipment using R-404A and correctly sized regular pilot-control valve can move to High Pressure control with no change. as checking the good restarting must be made during the time the high pressure goes down to its minimal value. Flash gaz Usually flash gas does not happen when High Pressure control is correctly implemented. This phenomenon (when associated with the HP control) may arrive in semi-season when the installation is operating at low power and condensers fans at full mode. This involves a small temperature difference of between condenser and outside. Consequently, added cooling is too small to compensate the pressure losses through the valve. Moreover the temperature of the room where is located the liquid container can reduce added cooling. To avoid this phenomenon it is necessary that the regulation limits the temperature difference between outside temperature and condenser. Other solution is to use that supercooling as a datum in the HP control to pilot the regulation. Installation of an external supercooling is an other solution to solve this problem. Refrigerant accumulation During winter time, HP control tends to accumulate more liquid in the condenser, especially if the gravitating flow of this one is not efficient. If the liquid container cannot store the difference between summer and winter, it will be necessary to re-consider the exhaust flow, the size of the bottle or to limit the minimal value of the high pressure. Discharge temperature High Pressure reduction involves a reduction of discharge temperature. This reduction can intervene on the performances of the oil separator. It will be necessary to make sure that the difference between condenser and discharge temperature is large enough to allow a correct separation. Impact on the heat recovery Heat recovery is more and more implemented on cold stores. It is done at several places of the installation: cooling; condensation; oil cooling; cylinder heads cooling. HP control and heat recovery are not inevitably incompatible. These two solutions bring pretty large energy savings. It is thus necessary to carry out studies on a case-by-case basis to find adjustments allowing to reduce total consumption. System instability For various reasons the system can become unstable in LP or HP. Regarding HP side, instability leads to frequent startings of fans, which can reduce motors life time. This dysfunction may have 2 origins: An incorrect adjustment of the PID loops (or another kind of regulation). In this case, it is just necessary to make a correct adjustment, Not enough regulation stages of the fans. Work on the PID can reduce the problem. It might be necessary to use more ventilation stages or a variable speed drive. Regarding LP side, frequent starting cycles of the compressors strongly increase the risks of breakage. The regulation cannot really be blamed since normally it was not changed. When the HP is reduced, the compressors provide much more refrigerating power. Operating time between 2 thresholds is thus reduced. When the number of compressors is small, this problem is frequent. Use of a variable speed drive is thus a perfect solution. Oil flow reduction With screw compressors, oil is generally put in circulation by the difference in pressure between the HP and LP (at least 4 bars). It will be necessary to maintain this difference of pressure even with a HP regulation. The pressure valve will have to be adjusted to a value equal or lower than the high pressure. It is also necessary to take into account the possible stops of the installation where the pressure gap will be reduced. Adjustment of the valve as well White paper on Energy Efficiency 5

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