Psychrometric analysis Revision of principles - Central plant for UBCLQT-40-3: Building & Engineering Systems Performance and UBLLSM-20-3: Mechanical Services

Psychrometric quantities temperature dry bulb, t [oC d.b.] wet bulb, t’ [oC w.b.] relative humidity,  [%rh] = avp / svp percentage saturation, μ [% sat.] = ms/mss moisture content, x [kg.kg-1dry air] specific volume, v [m3.kg-1 dry air] = 1/ specific enthalpy, h [kJ.kg-1 dry air] dew point temperature, td [oC]

Psychrometric chart Lines of constant d.b. temp. Dry bulb temperature, t

Psychrometric chart Lines of constant moisture content Moisture content, x

Psychrometric chart percentage saturation ,  Lines of constant relative humidity,  (percentage saturation)

Psychrometric chart Wet bulb temperature, t’ Lines of constant w.b. temp. Wet bulb temperature, t’ 5oC wb 5oC db

Psychrometric chart Lines of constant specific volume specific volume, v

Psychrometric chart Specific enthalpy, h ‘Lines’ of constant

Psychrometric chart Dew point temperature, td oC

State point Any two psychrometric quantities can be used to define a state point. All other properties can be read from the chart 50 %rh 26.5 kJ.kg-1 0.82 m3.kg-1 8.9oC db 0.00495 kg.kg-1 14oC db

Exercise - complete Table 1

Exercise – Solutions

Ideal processes Isothermal humidification Adiabatic humidification Sensible heating Sensible cooling Adiabatic dehumidification Isothermal dehumidification

Real processes - Sensible Heating Heating load, QH = ma (h2 - h1) h2 - h1 1 2  Heater Battery

Heating load, QH = ma (h2 - h1) Real processes - Sensible Heating Plant Loads Example 1: Air flows at a rate of 3.42 kg.s-1 into a heater battery. If the initial condition of the air is state 5 (see Table 1) and the air leaves the heater battery at a temperature of 31 oC d.b., determine the load on the heater battery in kW. Heating load, QH = ma (h2 - h1)

Real processes - Sensible cooling Cooling load, QC = ma (h1 - h2) (Nominal) h1 - h2 2 1  Cooler Battery

Real processes - Cooling & Dehum. 1 2  Cooler Battery

Real processes - Cooling & Dehum. Cooling load, QC = ma (h1 - h2) (Nominal) h1 - h2 1 2  Cooler Battery

Real processes - Cooling & Dehum. Cooling load, QC = ma (h1 - h2) (Nominal) h1 - h2 1 2  Cooler Battery

Contact Factor Cooling load, QC = ma (h1 - h2) h1 - h2 1 2 s ADP t

Real processes – Cooling with dehum Plant Loads Example 2: A cooler battery takes a flow of 1.5 kg/s of air at 25oC d.b. with a moisture content of 11.3 g/kg and dehumidifies to obtain a moisture content of air leaving the battery of 8 g/kg. If the cooler contact factor is 85%, determine the apparatus dew point and the nominal cooling load in kW. Cooler Contact Factor,

Real processes - adiabatic humidifier Adiabatic process - where no energy transfer occurs i.e. h1 = h2  Load = 0 2 1 Air Washer

Real processes - steam injection  h 2 1 Steam humidifier

Humidifier saturation efficiency ADP 2 1

Humidifier saturation efficiency, Real processes – Humidification Plant Loads Example 3: Air enters an air washer at 31oC d.b., 14.4oC w.b. and leaves with a moisture content of 0.008 kg/kg d.a. Determine the saturatiion efficiency of the air washer. What factors influence the saturation efficiency of humidifiers generally? Humidifier saturation efficiency,

Room heat loads Summer room loads:- Sensible heat gain + Latent heat gain Sensible Heat Ratio 1 Room ratio line R S tR - tS

Room heat loads Winter room loads:- Sensible heat loss + Latent heat gain Sensible Heat Ratio 1 Room ratio line R S tS - tR

Room heat loads Example 4: The room is to be kept at state 4 using a flow of 1.50 kg.s-1 of air at state 6. Determine the room sensible and latent loads in kW. The specific heat capacity of moist air is 1.02 kJ.kg-1K-1

Central plant AHU - 100% fresh air Summer cycle 2 4 6

Central plant AHU - 100% fresh air Summer cycle 2 Cooler battery 8 4 6

Central plant AHU - 100% fresh air Summer cycle 2 Cooler battery + Re-heater battery 8 4 6

Central plant AHU - 100% fresh air Summer cycle O Cooler battery + Re-heater battery C R S Room ratio line (RRL)

Central plant AHU - 100% fresh air Winter cycle 3 7 5

Central plant AHU - 100% fresh air Winter cycle Pre-heater battery 3 7 5 10

Central plant AHU - 100% fresh air Winter cycle Pre-heater battery + steam humidifier 3 9 7 5 10

Central plant AHU - 100% fresh air Winter cycle Pre-heater battery + steam humidifier 3 + re-heater battery 9 7 5 10

Central plant AHU - 100% fresh air Winter cycle Pre-heater battery Room ratio line (RRL) + steam humidifier R + re-heater battery B S O A

Central plant AHU - 100% fresh air Winter cycle Pre-heater battery Room ratio line (RRL) + adiabatic humidifier R + re-heater battery B S O A

Mixing AM A xA BM M xM B xB tB tM tA

Real processes - Mixing Example 5: Air in state 3 is mixed with air in state 4 to obtain mixed air, state M, with a dry bulb temperature of 25oC. If the mass flow rate of mixed air is 1.45 kg/s, determine the flow rates of consituent air flows.

Central plant AHU - recirculation air 2 6 4 C C’ M Summer cycle

Central plant AHU - recirculation air S M Room ratio line (RRL) Winter cycle