Energy-efficient refrigeration scheme with water-to-water chiller and free-cooling system

Among the energy-efficient solutions in refrigeration systems operating all year round, the most obvious is the use of freecooling (from English free cooling, literally translated as free cooling), which is designed to cool the intermediate coolant in a dry cooler, without using the refrigerant circuit during the cold season.

The use of such a scheme, when the refrigerant circuit is used in the warm season, and in the cold free-cooling has several significant advantages:

• Significantly reduced electricity consumption during the cold season;

• The service life of chiller compressors is increased;

• There is no limitation of the system operation by the minimum outdoor temperature.

At an outdoor air temperature below -15°C, such a system works more stable and eliminates the «heavy» operating conditions of the chiller.

The approximate payback period of a free-cooling system depends on capital costs, the time of using natural cold, the difference in power consumption, electricity tariffs and other factors. The validity of using freecooling in a refrigeration system largely depends on the number of days in a year with an average daily temperature below the coolant temperature for a given area. An example for some Russian cities from SNiP 23-01-99 «Construction climatology»:

City	length of day, with average daily air temperature
	≤ 0 °С	≤ 8 °C	≤ 10 °C
Arkhangelsk	177	253	273
Yekaterinburg	168	230	245
Irkutsk	177	240	258
Kazan	156	215	229
Moscow	145	214	231
Murmansk	187	275	302
Nizhny Novgorod	151	215	231
Novosibirsk	178	230	243
St. Petersburg	139	220	239

The time of using freecooling depends not only on the street temperature, but also on the circuit design. Consider the most commonly used schemes.

Scheme 1. Outdoor chiller with dry cooler.

The coolant is a glycol solution. If it is impossible to use it in the entire system, then the circuit is divided by an intermediate heat exchanger into water and glycol circuits with a pumping group in each.

Warm period of the year (Fig. 1).

The refrigerant through a three-way valve is sent to the evaporator, where it is cooled to a predetermined temperature. That is, only the chiller is used.

Transition period (Fig. 2).

The refrigerant through a three-way valve is first directed to the dry cooler, where it is partially cooled, then to the evaporator, where it is additionally cooled to a predetermined temperature. That is, the refrigerant circuit and freecooling are used together.

cold period (Fig. 3).

The refrigerant is sent through a three-way valve to the dry cooler, where it is cooled to a predetermined temperature, then to the evaporator. In this case, the compressors do not turn on because the set temperature has been reached. That is, only freecooling is used.

Such a scheme has good energy efficiency indicators, however, the use of an outdoor chiller is not always possible.

Scheme 2. Indoor chiller, dry cooler and intermediate heat exchanger (two modes of operation).

Warm period of the year (Fig. 4).

Water is sent to the evaporator by three-way valves, where it is cooled to a predetermined temperature. The heat of condensation is transferred by the glycol solution through the three-way valves to the dry cooler and vented to the atmosphere. That is, only the chiller is used.

cold period (Fig. 5).

Water is directed by three-way valves to an intermediate heat exchanger, where it is cooled to a predetermined temperature. Three-way valves direct the dry-cooled glycol solution to an intermediate heat exchanger to cool the water. That is, only freecooling is used.

Such a scheme is quite efficient and simple, but with its application it is impossible to share the refrigerant circuit and free-cooling during the transition period.

Our company has developed a freecooling refrigeration system scheme based on the Aircut water-to-water chiller, which allows the joint use of the refrigerant circuit and freecooling during the transition period.

The system includes the following main elements: chiller (water-to-water), dry cooler, intermediate heat exchanger and two pump groups. The first pumping group is on the side of the chilled water in the circuit of cold consumers, the intermediate heat exchanger and the chiller evaporator, the second is on the side of the glycol solution in the circuit of the dry cooler, the intermediate heat exchanger and the chiller condenser. The refrigeration system includes an integrated automation that provides fully automatic control and regulation.

Scheme 3. Indoor chiller, dry cooler and intermediate heat exchanger (three modes of operation).

one — chiller 2- dry cooler 3- Intermediate heat exchanger four — Controller 5 — bypass valve 6- 2-way valve — condensing pressure regulator as part of the chiller 7- 3-way valve for switching to «freecooling» mode eight — 3-way valve for regulating the performance of the intermediate heat exchanger

SWone — chilled water temperature sensor SW2- return water temperature sensor SW3- temperature sensor of the glycol solution at the inlet to the intermediate heat exchanger Sa — outdoor temperature sensor

Warm period.

Water circuit.

When the ambient air temperature (according to the Sa sensor) is higher than the return water temperature (according to the Sw2 sensor), water through the three-way valve enters directly into the chiller evaporator, where it is cooled to a predetermined temperature, and then sent to the cold consumers.

Glycol Circuit.

The glycol solution is cooled in the dry cooler, passes through an intermediate heat exchanger without heat exchange and is sent to the condenser and removes the heat of condensation, and then is pumped back to the dry cooler for cooling.

Transition period.

Water circuit.

If the outdoor air temperature (according to the Sa sensor) falls below the return water temperature (according to the Sw2 sensor), the flow of this liquid through the three-way valve (7) enters the intermediate heat exchanger, where it is cooled with a glycol solution. In this case, the water temperature does not reach the set value. Next, the water is sent to the chiller evaporator, where it is cooled to a predetermined temperature. The refrigeration unit in this case does not operate at full capacity, which will decrease as the outside air temperature drops until the moment of complete transition to free cooling.

glycol circuit.

The glycol solution is cooled in a dry cooler and sent to an intermediate heat exchanger, where it cools the water in the consumer circuit by 1–4°C, and then to the condenser. Since the liquid temperature at the condenser inlet is low, the condensing pressure is controlled by the two-way valve (6). Part of the flow is sent to the condenser and removes the heat of condensation, and part passes through the bypass line with bypass valve (5).

Cold period.

Water circuit.

The outdoor air temperature (according to the Sa sensor) is lower than the return water temperature (according to the Sw2 sensor), the water flow through the three-way valve (7) enters the intermediate heat exchanger, where it is cooled with a glycol solution to a predetermined temperature. Then it passes through the chiller evaporator without heat exchange process and goes to cold consumers.

glycol circuit.

The glycol solution is cooled in a dry cooler and sent to an intermediate heat exchanger, where it cools the consumer circuit water to a predetermined temperature. Then, through the bypass line with bypass valve 5, it enters the dry cooler.

At low outdoor air temperatures, in order to maintain the set temperature of the cooled liquid (according to the Sw3 sensor), the air flow through the dry cooler heat exchanger is reduced by changing the fan speed.

At very low ambient temperatures and zero air flow through the dry cooler (fans off), the coolant outlet temperature may be lower than the set temperature. The three-way valve (8) mixes the flow from the dry cooler and the flow from the pumps through the bypass line bypassing the dry cooler. Next, the flow of glycol solution with the required temperature (above 0°C) is sent to the intermediate heat exchanger. In this way, reliable operation of the plant is ensured at very low ambient temperatures without the danger of defrosting the intermediate heat exchanger.

Source: LLC «Airkat Klimatekhnik»