Design of refrigeration supply for ventilation systems

In the course of air conditioning engineering it is necessary to consider the properties of the premises, total loading of the cold supply system and the resources to drive the refrigeration station. Air conditioning is the steam air treatment what means not only cooling but also air wetting/drying depending on the process requirements to the treated premises.

There are three different solutions. Each solution may be implemented depending on the total required refrigeration performance:

• Air split systems up 30 to kW;
• VRF-systems from 30 to 150 kW;
• Centralized refrigeration stations based on freezing machines with the use of an intermediate refrigerant.

In the course of engineering space-planning parameters shall be also considered including the seasonal requirements of the refrigeration systems. All the above mentioned systems have their own advantages and disadvantages.

In its turn central freezing stations based on refrigeration machines may be divided into electricity driven vapor compression units and units which use heating – absorption refrigerating machines (driven by hot water, burning gas, smoke waste gas, etc.). Unique steam-ejection and air-driven units didn’t gain much popularity in the sphere of air conditioning.

There are four main solution based on vapor compression refrigerating machines:

1. Machines for internal mounting with outboard air-cooled condensers. Water is used as a coolant with the temperatures +7/+12°C. Outboard condensers shall be installed in the open air.

Advantages of such a system are the opportunity to use water as a coolant, a lower energy consumption, a small number of additional equipment.

Disadvantages are the necessity to install units in a heated premise and limitations on the height of the coolant.The solution is efficient for air conditioners equipped with the finishers (fan coils and chilled beams) and cooling sections of the plenum ventilation systems.

1. Mono-block machines for external installation. The refrigerant loop is inside the unit. Such machines are often installed on the roofs. The external way of installation is engineered to use salt solutions as a cooling agent due to their lower freezing temperature compared to water. This solution is recommended in case air is cooled in central ventilating units. A lower thermal capacity of the salt solutions requires the use of ventilating units equipped with larger cooling sections. This option is not recommended to use for the cold supply of inner finishers. While the aqueous solution of ethylene glycol is not allowed to use as a coolant due to its toxicity and for security reasons then it is necessary to come to use a more expensive aqueous solution with lower thermal characteristics, for example, propylene glycol solution or add an extra intermediate refrigerant loop such as a plate heat exchanger. Should a heat exchanger be used there is an aqueous solution of ethylene glycol circulating between the refrigerators and the intermediary heat exchanger and water circulating between the intermediary heat exchanger and end units. The use of the option results in the increase of the auxiliary equipment (pipe equipment, fixtures). In order to receive the water temperatures of +7/+12°C the chillers shall be working at a lower temperature range of the first loop (generally +5/+10) what results in cutting the boiling temperature in the evaporators. Thus the power consumption rises making the use of refrigerators with a higher performance more relevant. The use of auxiliary equipment results in boosting the capital costs.

Advantages of the system are the single option in case of sectional refrigerating, installation of large-sized equipment on the roof only, the use of chillers with the free-cooling system integrated.

Disadvantages which appear when working with the finishers are the big number of station elements, a higher power consumption, the necessity to have a heated area for a hydro kit and heat exchanger.

1. Mono-block units for internal installation equipped with water-cooled condensers and outboard dry chillers. Water is used as a cooling agent in this option. The intermediate circuit with the heat transfer agent (ethylene glycol aqueous solution) is used for heat removal into the environment. The intermediate loop increases the refrigerant condensing temperature in chillers compared to option 1.1. what results in the use of more powerful machines. The energy consumption is also boosting.

Pluses are the use of water as a coolant, the possibility of using “free cooling” in case the system is equipped with an intermediary heat exchanger. Another advantage is no limits in the way the dry cooler and chillers are placed.

Minuses are the big number of units used and a higher powerconsumption.

1. Mono-block units for internal installation equipped with water-cooled condensers, outboard coolers (ventilator cooling towers). The system differs from the previous option by the possibility of receiving lower condensing temperatures due to the evaporating cooling ability. Water serves as a heat transfer agent what allows the power consumption to be reduced.

The main advantages of the ventilator cooling towers is in the necessity of constant feeding, a cyclic replacement of the heat carrier due to the constant boost of the diluted salts, necessity to mount the cooling towers above the hydro kits. Moreover, the cooling towers become weighty due to the pallets and extended surface.

Refrigerating machines may also have different compressor types (extensive or dynamic principle of operation). They are distinguished by the principle of adjustment (stepless control and stage control methods) and by the cooling agent. Secondary energy resources of high grade allow absorption refrigerating machines to be used as a rationale. This equipment may also be used when there is a lack of power supply.

Advantages are in low power consumption and long operational life that makes up to 30-40 years.

Disadvantages are the dimensions, the necessity to use cooling towers of the open-type, necessity to use a boiler plant in the warm season.

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