3. An Air Handling Unit maintains a zone of a building at 20°C throughout the year and 50% saturation for the summer design case. The zone has a sensible heat gain of 100kW in the summer and a sensible heat loss of 30kW in the winter. The zone has the same constant latent heat gain in the summer and winter. The external design states are 27°C dry bulb and 21°C wet bulb in summer and -5°C saturated in the winter. Re-circulated air is mixed with fresh air in the ratio 3:1 by mass before entering the AHU which has a cooling coil and fan. The cooling coil has an ADP of 9°C and a contact factor of 0.80. The temperature gain across the fan is negligible. Using a psychrometric chart, calculate the: i) Latent heat gains in the zone (3 marks); ii) Cooling coil and re-heater loads for the summer design case (10 marks); iii) Room supply temperatures in winter (3 marks); iv) Total heating load in winter (4 marks);
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Since the sensible heat gain is given as 100 kW, we can assume that the constant latent heat gain is also 100 kW. ii) To calculate the cooling coil and re-heater loads for the summer design case, we need to consider the sensible heat gain, the ADP of the cooling Show more…
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A 100% outdoor summer air conditioning system has a room sensible heat load of 400 kW and a room latent heat load of 100 kW. The required inside conditions are 24°C and 50% RH, and the outdoor design conditions are 34°C and 40% RH. The air is supplied to the room at a dry bulb temperature of 14°C. Find a) the required mass flow rate of air b) moisture content of supply air, c) Sensible, latent heat loads on the coil, and d) The required cooling capacity of the coil, Coil Sensible Heat Factor and coil ADP if the bypass factor of the coil is 0.2. Barometric pressure = 1 atm. Comment on the results.
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Madhur L.
For a refrigerator or air conditioner, the coefficient of performance $K$ (often denoted as COP) is, as in Eq. (20.9), the ratio of cooling output $\left|Q_{\mathrm{C}}\right|$ to the required electrical energy input $|W|,$ both in joules. The coefficient of performance is also expressed as a ratio of powers, $$ K=\frac{\left|Q_{\mathrm{C}}\right| / t}{|W| / t} $$ where $\left|Q_{\mathrm{C}}\right| / t$ is the cooling power and $|W| / t$ is the electrical power input to the device, both in watts. (a) For a home air conditioner, $\mathrm{K}$ is generally determined for a $35^{\circ} \mathrm{C}$ outside temperature and an $27^{\circ} \mathrm{C}$ return air temperature. Calculate $\mathrm{K}$ for a Carnot device that operates between $35^{\circ} \mathrm{C}$ and $27^{\circ} \mathrm{C}$. (b) You have an air conditioner with $\mathrm{K}=3.2$. Your home on average requires a total cooling output of $\left|Q_{\mathrm{C}}\right|=1.9 \times 10^{10} \mathrm{~J}$ per year. If electricity costs you 12.5 pence per $\mathrm{kW} \cdot \mathrm{h}$, how much do you spend per year, on average, to operate your air conditioner? (Assume that the unit's COP accurately represents the operation of your air conditioner. A seasonal coefficient of performance $(S C O P)$ is often used. The $S C O P$ is calculated over a range of outside temperatures to get a more accurate seasonal average.) (c) You are considering replacing your air conditioner with a more efficient one with $\mathrm{K}=4.3$. Based on the value of $\mathrm{K}$, how much would that save you on electricity costs in an average year?
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