Chiller and water cooling system

Chiller and water cooling system is usually used in commercial building as a part of air conditioner system. Basically, chiller system consists refrigeration cycle which is used at common air-conditioner, it may be vapor compression or absorption. The difference between chiller and common air conditioner is what the machine cools. In common air conditioner, heat transfer occurs from air to evaporator (refrigeration system). But in chiller system, air is not cooled by evaporator. We can imagine how much refrigerant needed if we just use common air conditioner and how much power consumption for compressor. Chiller system uses refrigeration to cool water first, then cooled water will be pumped to Air handling unit (AHU).

In chiller system, there is heat exchanger which can be used to transfer heat from liquid to liquid, in this case between refrigerant to water. The common heat exchanger is shell and tube heat exchanger. Water temperature at outlet is colder than inlet because heat is transferred from water to refrigerant. This cooled water is pumped to Air Handling Unit (AHU). Heat transferred from room air occurs at this AHU. Air from AHU is cold and will be distributed by ducting to the room. 

Similar process is also used is water cooling system. In refrigeration cycle, condenser needs to release heat to ambient. Common air conditioner transfers heat from condenser to air. Condenser temperature is higher than outdoor temperature, thus heat transfer from condenser to the air can occur. This is why condenser is put outside. Basically we can put condenser inside, but it will make higher heat gain and it will be more difficult to cool the air.

In water cooling, condenser transfer heat to the water, Temperature of water at outlet is higher than inside. Outlet water from chiller (condenser) is pumped to cooling tower to be cooled, then this water flow to chiller again (condenser).  

Why is air in air-conditioned room dry?

When we are in the room or car which is air conditioned, we feel air surrounding us is dry. How come?

Air contains several kinds of gases such as nitrogen, oxygen, carbon dioxide, etc. Besides that air also contains moisture. If moisture in the air is high, then it makes air becomes more humid. The amount of moister in the air is called humidity. Humidity can be quantified as relative humidity and humidity ratio.

 

Latent heat is heat which doesn’t change the temperature. This heat only change phase. Moister inside air causes latent heat becomes high. More moisture contained in the air will cause higher latent heat because heat removed from air is not only used to decrease temperature, but also condense moisture.

Air-conditioner cool the air and it causes air temperature decrease until dew point temperature, if the air is still cooled, temperature will decrease followed condensation of moisture. Dew point is the temperature when moisture is saturated.

Room air is sucked to the air conditioner and blown again to the room. Before it is blown again, air is cooled by air-conditioner. In other words air is circulated and cooled by air-conditioner. The component of air-conditioner which cool the air is evaporator, thus air which is close to evaporator is colder than other and achieve dew point temperature earlier, then start to produce condensate. That is why water is often (condensate) found at air conditioner.

Since moisture in the air condenses at evaporator, the amount moisture is being reduced and resulting dry air. 

Air conditioning and Ventilation

Air conditioning

Air conditioning is treatment to make temperature, humidity, cleanliness of air meet the requirements of conditioned space. It also includes the distribution of air. Air conditioner can be divided in to several types, those are direct expansion coolers, chilled water system, and evaporative coolers.

 Direct expansion cooler is the kind of usual air conditioner that found in residential house. This air conditioner uses refrigeration cycle as its principle. Air is blown through the cold part (evaporator) and distribute to the room.

Chilled water system doesn’t  cool the air directly. This air conditioner cool water and then water will be flown to the Air Handling Unit (AHU) to cool the air.

Evaporative cooler is usually used in hot and dry climate region. This air conditioner evaporates the water sprayed and resulting lower temperature of air.

Ventilation

Ventilation is the process that removes or supplies air at the room. Every room has function, such as class room, laboratory, etc. Those rooms have different requirements of ventilation. Designing HVAC system consider many aspects of parameter, one of them is how much air must be supplied.  This parameter can be achieved by designing ventilation system.

All parameter HVAC design requirement are treated to make thermal comfort for the occupancy. ASHRAE made a standard about human comfort. (ASHARE STANDARD 55)

Refrigeration Cycle

Refrigeration cycle produces hot part and cold part (compare to ambient temperature). It needs at least four components, they are compressor, condenser, expansion device, and evaporator. Work fluid (refrigerant) goes around from one component to other component again and again, so we call it cycle. Refrigerant experiences all process in components

Figure 1. Refrigeration cycle schema

Figure 1 show schema of refrigeration cycle which consist of four main components and connected pipes (showed by line), and also the flow direction of refrigerant is showed at that figure. At the figure, there are two types of line, red and blue. The red one shows that the pressure is high and the blue shows the pressure of refrigerant is low.  As known, pressure is proportion by temperature, then the color of line also show the temperature condition of refrigerant.

1 to 2, Compression process. Output from evaporator, refrigerant is gas phase. Then compressor sucks it from evaporator and discharge it to condenser.  Before through the compressor the refrigerant pressure  is low and after through compressor refrigerant pressure is high. This process also makes the refrigerant temperature increase.

2 to 3, Condensation process. Output from compressor, refrigerant phase is still gas. As its name condenser is a component to make refrigerant phase change  from gas into liquid. This process needs to release heat. Usually condenser is put outside, because the temperature of ambient outside is lower than temperature of refrigerant in condenser, then heat transfer can occur from refrigerant to outdoor air temperature.

3 to 4, Expansion Process. Output from condenser, refrigerant phase  is already liquid and has  high pressure and temperature. Expansion device makes pressure of refrigerant drop into low pressure, this process also is followed by drop temperature. Most of refrigerant phase after through expansion valve is still liquid and a few gas.

4 to 1, Evaporation Process. As its name, the function of this component is to make refrigerant phase changes from liquid into gas. This process need heat, and heat is got from the objects which want to be cooled. The temperature of refrigerant in evaporator is lower than the objects, then heat transfer will occur from objects to refrigerant, then heat is used for evaporate refrigerant. After the refrigerant is already in gas phase, refrigerant  will experience the process from number   1 again. It happen repeat and repeat again.

Air Conditioner and refrigerator

Air conditioner and refrigerator have same work principle, it is refrigeration cycle. Refrigeration is reverse of Power cycle (rankine cycle) which use heat for produce some work (Power), refrigeration use  work to move the heat. If we read some books, maybe we will find that refrigeration cycle is a system which allow heat transfer from cold area to warm area (figure a) whereas heat transfer is spontaneously from warm area to cold area. Actually, if we focus on each component of refrigeration cycle, heat transfer from cold area to warm area is not happen. But because of we assume the refrigeration cycle as one system, we can say like that.

Fugure (a). refrigeration system as one system

Figure (b). refrigeration system produce cold and hot part

In fact, refrigeration cycle produce the difference of temperature, there are cold part and hot part (figure b). We use cold part to cool the objects. Object in this content is defined as all things that we want to make it cold. In refrigerator, the objects are foods, water, etc. But, in air conditioner system the object is the room air.

Refrigeration system at least needs four components, there are compressor, condenser, expansion valve, and evaporator. Refrigeration system also need work fluid, it is called refrigerant. There are many types of refrigerant, some of them are forbidden because of the environment issue. Refrigerant  always goes around from one component to another component, and at each component refrigerant experiences the process based on the function of component.