The following data is given for the reheat system shown below: Sensible load = 7

Question

The following data is given for the reheat system shown below: Sensible load = 700,000 Btu/hr, Latent load = 300,000 Btu/hr, Space (State 2 and 3): 77 degree F DBT, Outdoor (State 4): 95 degree F DBT, and 40% RH, Supply (State 1) air flow rate: 52800 cfm, Exhaust (State 3) airflow rate: 8000 cfm, Cooling coil exit (State 6): 50 degree F DBT, and 90% RH. Neglecting fan and duct heat gains, determine the supply air temperature, the relative humidity in the space, and the reheat coil and cooling coil capacities (in BTU/hr). Show the processes on the attached psychrometric chart. Label each state.

Explanation / Answer

solution:

1)here flow rate are related as

Qsupply=Qexhaust+Qreturn

Qreturn=52800-8000=44800 cfm or 1269 cmm

Qsupply=52800 cfm or 1495 cmm

here amount of exhaust air is equal to amount of fresh air

Qexhaust=Qfresh air=8000 cfm or 226.5 cmm

2)here Oouside air sensible heat load is

OASH=.0204*Qfresh*(To-Tr)

To=95 F or 35 c

Tr=77 F or 25 c

OASH=46.206 kw

3)here outside air latent heat load is given by

OALH=50*Qfresh(Wo-Wr)

from psychrometric chart

Wo=.014 kg/kg of dry air

where cooling coil inlet temperature is given by

Te=Qfresh*To+Qreturn*Tr/(Qreturn+Qfresh)

Te=26.51 c

Tl=10 c and 90 RH or .007 kg/kg of dry air

as cooling coil just provide sensible cooling hence specific humidity at inlet and outlet remain same

We=.007 kg/kg of dry air

where from chart ,we are provide sensible cooling and reheating hence supply air specific humidity is remain same

Wr=.007 kg/kg of dry air

nowoutside air latent heat load

OALH=50*Qfresh*(Wo-Wr)

OALH=79.2687 kw

4)now total sernsible and latent heat load are

SH=RSH+OASH=205.1497+46.206=251.35 kw

LH=RLH+OALH=87.9213+79.2687=167.19 kw

5)cooling coil load is

cooling coil load=SH+LH=418.54 kw

where available capacity=.0204*Qsupply(Tl-Te)=503.52 kw

6) as cooling coil provode additional cooling hence this much load has to supply by reheater

reheater capacity=available cooling coil capacity-cooling coil load=84.98 kw

7)air supply temperature is

reheater capcity=.0204*Qsupply(Trs-Tl)=.0204*1495(Trs-10)=84.98 kw

Trs=12.78 c

8)where room relative humidity is varying and it is

for 25 c saturated vapor pressure is

Pvs=.03166 bar

P=1 atm=1.013 bar

wr=.622(Pv/P-Pv)

we get for Wr=.007

Pv=.01127 bar

hence relative humidity=RHr=Pv/Pvs=.01127/.03166=.355969 or 35.59%

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