Commercial Refrigeration Equipment
HP control consists in regulating the condensation pressure at a given value in order to obtain the lowest power consumption of the compressor / condenser couple (and auxiliaries)...
A refrigeration system is a thermodynamics cycle, which transports heat from a cold storage, via an evaporator, to the outside via a condensor (Figure 1 shows a refrigeration system in order to locate devices).
To understand succintly the benefits and operation of HP control, it is not necessary to fully understand the operation of the refrigeration installation.
The compressor is the heart of the circuit, as it compresses the gas generating the flow necessary for the cycle. Generally, the compressor consumes the major portion of the energy. Its consumption is not constant and depends on several variables, most important are the low and high pressures. Some compressors are equipped with a mechanical device to reduce cooling capacity. The use of these partial load devices affects the compressor efficiency.
In terms of energy consumption, the most useful is the COP (Coefficient Of Performance). The COP takes into account variation of internal compressor efficiencies and the refrigeration cycle status. It is therefore necessary to have the operating status associated with the COP to be able to judge. (Example: -10 °C / +35 °C).
COP is the ratio of the cooling capacity produced (or useful) to the consumed electrical power. The COP operates in the same direction as efficiency.
There are several types of the compressors, most representative are:
• recriprocating compressors
• scroll compressors
• screw compressors.
The following explanation is applicable to these three types of compressors.
Note : there are some specific characteristics on certain compressors.
Fig. 1: Representation of a refrigerating system...
The condenser’s function is to dissipate calories. It is usually on the roof or outside. It can be used to heat water for another use.
We can distinguish four categories of condensers:
• evaporative condensers
• adiabatic condensers
• hybrid condensers.
• lost water condensers
• opened circuit cooling tower
• closed circuit cooling tower
• hybrid cooling tower
• dry air cooler
• adiabatic air cooler
• heating networks or intermediate heating networks.
Evaporative condenser or other gas heater
HP control can be applied to all these condensers (except evaporative condensers and heating networks), but explanations given in this document are primarily applicable to dry air coolers and condensers.
There are some adaptations needed to make it applicable to other condensers.
HP is created by the balance between the heat to be dissipated in red on the chart, and cooling capability in green.
The system must dissipate a quantity of heat, which depends on the instantaneous cooling power and the compressors efficiency.
The condenser can dissipate a certain amount of calories depending on its operating conditions: a large temperature difference between cooler and fluid wil increase the cooling capability.
Fig. 2: High pressure according to the evacuable powers...
Pressure can also be derived from the saturation temperature (temperature from which the liquefied refrigerant evaporates or the gaseous refrigerant condenses). This temperature increases as the pressure increases. According to the fluid in the system, a HP at 40°C will not have the same pressure.
On the graph, it is clear that when the HP temperature is equal to the outside temperature, heat which can be dissipated is equal to zero.
The highest HP temperature the highest is the power that can be dissipated (temperature difference between outside and the fluid temperature is high). In other words heat rejected by the condensor (in green) increases.
For the compressors (in red), when the HP increases, power which is disspated increases too but slower.
Steady state for HP, is when the heat produced by the compressor and the heat rejected by the condensor are identical.
Fig. 3: Flow influence of the fans...
In order to control this balance, the condenser capability is adjusted by controlling the cooling fans. Increasing the amount of airflow across the condenser increases the performance of the condenser and vice versa as shown figure 3.
Of the many variables that effect the heat dissipation of the condenser, the only one we can control is the airflow trough the condenser
HP control consists in regulating the condensing pressure value to obtain the lowest consumption of the compressor/condensers couple (and auxiliaries).
This is definitely not to reduce HP to the minimum.
HP control modes
The implementation of the HP modes is not identical with all condensers. It is understandable that the control is not implemented or controlled in the same way with a dry condenser or a cooling tower. However, the methods described below are applicable with some modifications.
Constant HP or hysteresis control
This method is the most used control method, however, with implementation of HP control being easier and for other added benefits, this method is slowly being replaced. The goal is to maintain HP at a constant value that can be held throughout the year. For a constant HP, it would be necessary to use a regulation with neutral zone or a PID. However most common solution is the use of pressure switches or hysteresis controller creating steps in the HP regulation (Figure 4).
HP is not really regulated at a constant value, it will vary uncontrolled according to the outside temperature, the heat to be dissipated and the number of fans required to accomplish this operation.
Fig. 4: HP variation according to the number of fans...
Reducing HP is interesting in terms of energy consumption: when HP decreases the compressor COP increases, and vice versa. Figure 5 shows the COP as a function of the condensation temperature for a screw compressor, COP variation is clearly visible. In the example, it jumps from 1.9 at -10 °C / +50 °C to 4.7 at -10 °C / +20 °C i.e. a variation of 62%.
Figure 6 gives the percentage gain (or loss) on COP for variations of one degree of condensation temperature (given in Kelvin) according to the HP and for various evaporation temperatures. All compressors do not react the same way, it is therefore necessary to use the characteristics of the actual compressors to correctly assess the energy savings.
However, to reduce HP, it is mandatory to operate more fans. Energy savings is thus less than those calculated for the compressor.
It is necessary to calculate the COP on the compressor and condenser as a whole to specify HP control.
The use of fans should be made whith definite purpose and with absolute need. Sometimes savings made on the compressor can be offset by the use of fans.
The graph of Figure 7 demonstrates the existence of an optimum. This phenomenon often occurs on installations operating below 50% of full load.
To summarize, HP control consists in regulating the condensation pressure at a givenvalue in order to obtain the lowest power consumption of the compressor / condenser couple (and auxiliaries).
This is definitely not to lower the maximum HP, which could on top of an increase of the power consumption cause malfunctions of the installation. (See appendix)
Fig. 5: COP variation vs HP for a screw compressor...
Fig. 6: COP variation (K in%) vs HP...
Concretely on the field
Installation is relatively simple (see example in figure 8). The controller embedded with HP control algorithms, receives HP information of the refrigerant, the outside temperature and then, processes this information.
The controller converts pressure to temperature (depending on refrigerant fluid used).
It calculates the differential with the outside temperature.
This differential is the parameter to be controlled. A PID function is used (it is a control block) that gives the percentage of condenser power.
This percentage is translated into the number of fans required.
Fig. 7: Powers optimization of the compressor/condenser...
Application on storage system
Example of calculation on a cold store, a fixed HP at 40°C is compared to a HP control. The comparison is done for 2 outside temperatures: 30°C and 15°C.
When the outside temperature is high, energy savings are low, even nonexistent. Once the outside temperature decreases, energy savings increase strongly. It should be noted that in France average temperature is around 11°C far from 30°C. Savings are consistent, but dependent on several factors.
Fig. 8: Installation example...
This example will help in showing the impact of two factors - the outside temperature and the load of the installation – which influences the performance of installation and those of the HP control.
This nstallation, produces 500 kW of cold when running at its maximum speed, i.e., a Low Pressure (LP) at -10°C and a HP at 50°C. Compressors have a COP rated 3.4 at -10 / +30°C. The condenser ventilation power is 40 kW; which dissipates 685 kW with a differential of 10°C. The minimum HP temperature is limited to 20°C for technical constraints.
Figures 9 and 10 give the power consumption of the compressor and condenser for different outdoor temperatures and regulation requirements. Each curve represents the power absorbed by the compressor and condenser for several external temperatures.
X-coordinate is the difference between the outside temperature and the HP. The addition of the value in X-coordinate and the outside temperature gives the HP value.
This chart can be used to define what algorithms is the best suited to reduce the power requirements to the minimum.
In this example, when the installation operates at full cooling capacity (Figure 9), that is to say 500 kW of cold, running all fans is less energy demanding whatever the outside temperature.
Fig. 9: Electric output of the whole at 500kW...
Fig.10 Electric efficiency of the whole at 500kW...
When the installation is running at partial load i.e., 40% load (Figure 10), input power decreases with the reduction of HP. From the optimal HP, power consumption increases while HP continues to decline. Savings are about 1.5% / K at right of the optimal HP and -1.5% / K at the left of the optimal HP. These values are not generic for all installations.
An optimal value of HP emerges: the goal of a HP control will be to regulate the installation at this value.
Note: that these values are for a given installation, it is necessary to analyse each installation to determine the optimum HP.
In today’s climate, energy saving solutions are a must. Environmental aspect is sometimes not sufficient to justify the huge required investments.
Solutions as HP control have the benefit of reducing the environmental impact not to mention the financial aspect.
HP control remains an effective and current solution for energy savings. There may be differences between solutions and their implementations.
To improve the return on investment, good commissioning must not be forgotten.
This solution, according to the installations, is not very expensive; however it can have very significant energy savings, exceeding 30%. HP control is the solution with the best return on investment for refrigeration.
Today, all new installations must have an effective HP control.
Source: Schneider Electric India
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