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Emerson
 
| In any standard fixed capacity a/c machines – split, packaged, chiller etc, the output from the machine is always either “full capacity” or “no capacity”. This is a function of the compressor switching “On” when there is full capacity output and switching “Off” when there is zero capacity output. For any space that requires air-conditioning, the room load increases as the ambient temperature increases. The following chart explains this concept in simple terms. The x axes represent the ambient temperature and the y axis shows the cooling/ heating demand. The dotted line shows the room demand. |
Fixed Speed Systems for Cooling Application
Figure 1 click to view
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Figure 2 click to view
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As the ambient temperature rises above 20 deg C, the cooling need increases in the room and similarly as the temperature outside starts dropping below 20 deg C, the heating demand starts rising. (Figure 1).
The capacity output from a fixed speed standard machine is shown by the dark blue lines. In the cooling cycle, as the ambient starts increasing, the cooling output from the fixed speed machines start dropping. This is intuitive, as we know that during the hot summers, as the outside temperature increases, the cooling output from the a/c machines starts dropping. This phenomenon is also known as de-rating of the a/c system. As can be seen from the graph, there is only one point where the solid blue line (representing the capacity output from the a/c) and the dotted line (representing room demand) matches. At all other points, there is a mismatch. At ambient above 35 deg C (as shown in the graph) the a/c machine is unable to provide the additional cooling that is required and so the red area represents the inability of the a/c machine to deliver adequate cooling to maintain the room temperature to the desired levels. In such conditions, the room will never attain the set point temperature and will always be warmer than required. (Figure 2).
Figure 3 click to view |
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| On the other hand, when the outside temperature starts to fall below 35 deg C, the machine operates in the yellow zone. In the yellow zone, the a/c is delivering more capacity than is required by the space and so the compressor periodically turns ON and OFF. Switching of the compressor ON and OF F is a waste of energy. The reason is that once the machine is turned off through the thermostat, the refrigerant pressures equalizes in the system and then additional energy is required to start up the compressor. (Figure 3). |
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Figure 4 click to view
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Figure 5 click to view
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The first situation is similar for the heating case. When the ambient starts to fall below – 5 deg C, the heat pump is unable to deliver enough heating capacity and the room temperature will be lower than the set point. But if the ambient temperature is above – 5 deg C, there is excess heating capacity from the heat pump and the compressor would again be switching On and Off and as explained before, this is an inefficient operation. (Figure 4 & 5). |
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Figure 6 click to view |
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| These are the 2 major values of capacity modulation: Energy savings achieved by ensuring that the capacity output matches the room demand precisely AND better human comfort through a tight temperature tolerance in the room.
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