Centralized Direct Expansion System

This is a direct expansion (DX) system using HFC for both the low temperature (LT) and medium temperature (MT) loads (i.e. frozen and chilled display cabinets respectively).

Comprising two completely separate systems (LT and MT) it is typical of systems being installed in supermarkets throughout Europe today. Each system uses a central “multi-compressor pack” consisting of between three to eight semi-hermetic reciprocating or scroll compressors. The pack is located in a plant room and is connected to an external air-cooled condenser. High pressure refrigerant liquid is fed to the cabinets in the store, usually via a liquid receiver (not shown). The refrigerant vapour then returns to the compressor pack via the suction line. The condenser pressure is typically controlled by varying the air flow. This ensures that the condensing pressure is kept as low as possible, consistent with satisfactory expansion valve control when outdoor air temperatures are low. The minimum condensing condition is assumed to be 20°C for air-cooled vapour compression circuits.
Pressure drops in lines may be accounted for by adjusting evaporating temperatures. The table below shows examples for the default values.

Refrig	Tair	TD (Air-Refig)	SST (Mid Pt)	∆T Pressure (Drop Effect)	SST (Mid pt)	SST (Dew pt)
R404A	-10	7	-17	2	-19	-19
R404A	33	10	23	3	20	20
R407A	-10	7	-17	2	-19	-14.6
R407A	33	10	23	3	20	24.3
R134a	33	10.9	22.1	3	19.1	19.1
R410A	-10	5.4	-15.4	2	-17.4	-17.4
R410A	33	8.4	24.6	3	21.6	21.6

Distributed Direct Expansion System

This system is similar to the centralised DX except that the compressors are usually located adjacent to, or within the condenser housing.

Instead of a central plant room, a number of smaller compressors are mounted within each condenser unit. The condenser units are usually roof mounted, with each unit positioned directly above a group of cabinets so that long refrigerant lines are avoided. The condenser units can be factory-assembled, which enables their quality to be closely controlled and streamlines the construction process, making it faster, more efficient, safer and less costly.This approach also reduces the refrigerant charge and leakage rate due to smaller diameter pipe sizing. However, it is not always practical; whether or not it can be used depends on the location and structure of the building. Pressure drops in lines may be accounted for by adjusting evaporating temperatures. The table below shows examples for the default values.

Refrig	Tair	TD (Air-Refig)	SST (Mid Pt)	∆T Pressure (Drop Effect)	SST (Mid pt)	SST (Dew pt)
R404A	-10	7	-17	1	-18	-18
R404A	33	10	23	2	21	21
R407A	-10	7	-17	1	-18	-13.6
R407A	33	10	23	2	21	25.3
R134a	33	10.9	22.1	2	20.1	20.1
R410A	-10	5.4	-15.4	1	-16.4	-16.4
R410A	33	8.4	24.6	2	22.6	22.6

Cascade CO₂ System

With this technology, an HFC centralised DX system is used for MT loads and the LT system has a separate circuit that discharges its heat into the suction stage of the MT system.

The LT circuit has a low condensation temperature so R744 (CO₂) can be applied in subcritical mode without excessive pressures. The challenges are not fundamentally different from systems with conventional refrigerants. The discharge pressure (at around 30 to 35 bar) is still within the normal design limits for refrigeration pipe work and components (typically 40 bar).The temperature difference required to drive the heat transfer across this extra heat exchanger represents a slight loss in energy efficiency compared to a DX system.
Pressure drops in lines may be accounted for by adjusting evaporating temperatures. The table below shows examples for the default values. The table below also shows examples for how to arrive at SST (Saturated Suction Temperature) by using TD’s (Temperature Differentials) for secondary systems.

Refrig	Tair	TD (Air-Fluid)	T (Fluid)	TD (Fluid Refig)	SST (Mid pt)	∆T Pressure (Drop Effect)	SST (Mid pt)	SST (Dew pt)
CO₂	-10	NA	NA	6	-16	1	-17	-17
R404A	33	10	23	5	18	0	18	18
R407A	33	10	23	5	18	0	18	22.3
R134a	33	10	23	5	18	0	18	18
R410	33	10	23	5	18	0	18	18

Secondary System

The table below shows examples for how to arrive at SST (Saturated Suction Temperature) by using TD’s (Temperature Differentials) for secondary systems.

Refrig	Tair	TD (Air-Fluid)	T (Fluid)	TD (Fluid Refig)	SST (Mid pt)	SST (Dew pt)
R404A	-10	6	-16	5	-21	-21
R404A	33	10	23	6	17	17
R407A	-10	6	-16	5	-21	-16.6
R407a	33	10	23	6	17	21.3
R134a	33	10	23	6	17	17
R410A	-10	6	-16	5	-21	-21
R410A	33	10	23	6	17	17

Secondary Glycol Systems :
A secondary refrigerant distributes the MT cooling around the supermarket. The LT arrangements are identical to the cascade system.Heat from the chill cabinets is transferred to the MT evaporator/heat-exchanger by circulating glycol as a secondary heat transfer fluid. Heat exchangers can be located close to the compressors and when assembled with an air cooled condenser this package can be delivered as a factory assembled unit similar to an air conditioning chiller. This results in a significantly reduced MT refrigerant charge and potential refrigerant leakage is reduced due to the factory manufacturing.Secondary refrigerant systems require a pump to circulate the coolant around the supermarket and there is an extra heat exchanger temperature drop. The heat exchanger requires a temperature difference to drive the heat transfer and so the MT evaporating temperature must be lower than the secondary refrigerant temperature, leading to an increase in compressor energy consumption. The secondary refrigerant temperatures and temperature differences used in this model are explained in the Energy section.Secondary Glycol systems require less refrigeration service effort than DX solutions.

Secondary CO₂ Systems :
CO₂ in the liquid phase distributes the MT cooling around the supermarket. The LT arrangements are identical to the cascade system.Heat from the chill cabinets is transferred to the MT evaporator/heat-exchanger by circulating CO₂ as a secondary heat transfer fluid. Heat exchangers can be located close to the compressors and when assembled with an air cooled condenser this package can be delivered as a factory assembled unit similar to an air conditioning chiller. This results in a significantly reduced MT refrigerant charge and refrigerant leakage is reduced due to the factory manufacturing.Secondary refrigerant systems require a pump to circulate the coolant around the supermarket and there is an extra heat exchanger temperature drop. The heat exchanger requires a temperature difference to drive the heat transfer and so the MT evaporating temperature must be lower than the secondary CO₂ temperature. However, the better heat transfer properties of CO₂ in the cabinets allows the circulating fluid to operate at -1°C instead of -3°C with glycol. In this model a temperature difference of 5K has been applied so that the MT evaporating temperature is -6°C.

Booster CO₂ System

This model uses R744 (CO₂) as a direct refrigerant in both the MT and LT systems. The LT compressors act as boosters to raise the vapour pressure to the level of the MT evaporators.

At ambient temperatures above approximately 23°C the compressors discharge the gas above the critical pressure of R744 (74 bar). The condenser then acts as a gas cooler and reduces the temperature of the discharge gas without condensing it into liquid. Cooled fluid passes through a pressure reduction valve, at which point a portion condenses into liquid and the rest remains as gas. Liquid and gas are separated in a flash vessel controlled by the pressure relief valve at a pressure of around 35 to 40 bar.The liquid is then distributed to the MT and LT cabinets via the liquid line at this intermediate pressure. The flash gas is taken via an additional expansion device to the suction of the MT compressors. A separate flash gas compressor may be an adequate method to raise system efficiency in warmer regions.
Pressure drops in lines may be accounted for by adjusting evaporating temperatures. The table below shows examples for the default values.

Refrig	Tair	TD (Air-Fluid)	SST (Mid pt)	∆T Pressure (Drop Effect)	SST (Mid pt)	SST (Dew pt)
CO₂	-10	6	-16	1	-17	-17
CO₂	33	9	24	1	23	23

The Systems

Centralized Direct Expansion System

Distributed Direct Expansion System

Cascade CO₂ System

Secondary System

Booster CO₂ System