R&AC Lecture 30 - Lesson 30 Psychrometry Of Air...

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Lesson 30 Psychrometry Of Air Conditioning Systems Version 1 ME, IIT Kharagpur 1
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The specific objectives of this lecture are to: 1. Purpose of psychrometric calculations ( Section 30.1 ) 2. Analysis of a simple, summer air conditioning system with 100% re-circulated air ( Section 30.2.1 ) 3. Analysis of a summer air conditioning system with outdoor air for ventilation and with zero by-pass factor ( Section 30.2.2 ) 4. Analysis of a simple, summer air conditioning system with outdoor air for ventilation and with non-zero by-pass factor ( Section 30.2.2 ) 5. Analysis of a summer air conditioning system with re-heat for high latent cooling load applications ( Section 30.2.3 ) 6. Selection guidelines for supply air conditions ( Section 30.3 ) At the end of the lesson, the student should be able to: 1. Estimate the load on the cooling coil and fix the supply conditions for various summer conditioning systems, namely: a) Systems with 100% re-circulation b) Systems with outdoor air for ventilation with zero by-pass factor c) Systems with outdoor air for ventilation with non-zero by-pass factor d) Systems with reheat for high latent cooling load applications . 30.1. Introduction: Generally from the building specifications, inside and outside design conditions; the latent and sensible cooling or heating loads on a building can be estimated. Normally, depending on the ventilation requirements of the building, the required outdoor air (fresh air) is specified. The topic of load estimation will be discussed in a later chapter. From known loads on the building and design inside and outside conditions, psychrometric calculations are performed to find: 1. Supply air conditions (air flow rate, DBT, humidity ratio & enthalpy) 2. Coil specifications (Latent and sensible loads on coil, coil ADP & BPF) In this chapter fixing of supply air conditions and coil specifications for summer air conditioning systems are discussed. Since the procedure is similar for winter air conditioning system, the winter air conditioning systems are not discussed here. Version 1 ME, IIT Kharagpur 2
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30.2. Summer air conditioning systems: 30.2.1. Simple system with 100 % re-circulated air: In this simple system, there is no outside air and the same air is recirculated as shown in Fig.30.1. Figure 30.2 also shows the process on a psychrometric chart. It can be seen that cold and dry air is supplied to the room and the air that leaves the condition space is assumed to be at the same conditions as that of the conditioned space. The supply air condition should be such that as it flows through the conditioned space it can counteract the sensible and latent heat transfers taking place from the outside to the conditioned space, so that the space can be maintained at required low temperature and humidity. Assuming no heat gains in the supply and return ducts and no energy addition due to fans, and applying energy balance across the room; the Room Sensible Cooling load (Q s,r ), Room Latent Cooling Load (Q l,r ) and Room Total Cooling load (Q t,r ) are given by: ) t t ( C m Q s i pm s r , s = (30.1) ) W W ( h m Q s i fg s r , l = (30.2) (30.3) ) h h ( m Q Q Q s i
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R&AC Lecture 30 - Lesson 30 Psychrometry Of Air...

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