| PERFORMANCE CALCULATIONS
FORMULAS:
Effectiveness = ( Actual Energy Transfer) /
(Max. Energy Transfer Possible)/
Expressed as: E = Wsup (X1-X2) / Wmin (X1-X2)
or E = Wexh (X1-X2) / Wmin (X1-X2)
Where:
E = Sensible, latent
or total heat effectiveness
X = Dry bulb temperature
(T), humidity ratio (Z) orenthalpy (h) at the location illustrated
Wsup = Supply airflow, SCFM
Wexh = Exhaust airflow, SCFM
Wmin = The smaller of Wsup or Wexh
Wmax = The larger of Wsup or Wexh
Osup = Heat recovered by supply air
Qexh = Heat recovered by exhaust air
Note:
A number of methods are available to
calculate seasonal savings and paybacks. One source is "Energy Recovery Equipment and
Systems; Air to Air," SMACNA, 1978, Section XIII and XIV.
PROCEDURE:
1 . For VHR unit size selected with equal
airflows, obtain effectiveness, face velocity and pressure drop from Table 2.
2. For VHR units with unequal airflows
obtain face velocity and pressure drop from Table 2 at actual exhaust and supply SCFM
each. Read equal airflow effectiveness from Table 2 at smaller airf low, then enter Table
3 with that value
Examples (Psychrometric
chart required)
Model VHRE (total heat recovery)
WINTER
Outdoor Air = 150° FDB, 70% RH
Exhaust Air = 74° FDB, 62° FWB
Supply and Exhaust Airflow = 3000 SCFM
VHRE Effectiveness (Table 2) = .75
Supply Air Leaving Wheel:
T2 = T,-E (Wmin/Wsup) (Tl - T3)
T2 = 15-.75 (3000/3000) (15 - 74) = 59.3 FDB
Z2 = Zi-E(Wmin/Wsup)(Z! - Z3)
Z2 = 8-.75 (3000/3000) (8 - 64).
Z2 = 50.0 Grains/lb.of dry air:; 52.4 FWB
Osup = 4.5Wsup (h2-h1)
Osup = 4.5(3000)(22.7- 4.7) = 243,000 Btuh
Exhaust Air Leaving Wheel:
T4 = T3-E(Wmin/Wexh) (T3 - T1)
T, = 74-.75 (3000/3000) (74 - 15) 29.8 FDB
SUMMER
Outdoor Air = 93° FDB, 75° FWB
Exhaust Air = 74° FDB, 62° FWB
Supply Air Leaving Wheel:
T, = 93 - .75 (3000/3000) (93 - 74) 78.8 FDB
Z, = 102 - ' 75 (3000/3000) (102 - 64)
Z2 = 73.5 Grains/lb. of dry air; 65.7 FWB
Osup = 4.5 (3000) (30.2 - 38.6)
Osup = - 113,400 Btuh (cooling)
HEAT
WHEEL
Leaving Air Conditions:
Formula 1: X2 = X1 - E(Wmin/Wsup) (Xl - X2)
Formula 2: X4 = X3 - E(Wmin/Wexh) (X3 - X1)
For Sensible Heat Recovery Wheel (VHRS):
Formula 3: Qsup = 1.08 Wsup (T2 - T1)
For Total Heat Recovery Wheel (VHRE):
Formula 4: Osup = 4.5 Wsup (h2 - h1)
Note:Qexh=-Qsup regardless of wheel typetype
and ratio of larger airflow to smaller
airflow to read corrected effectiveness.
3. Calculate leaving air conditions using Formulas 1 and 2.
4. Calculate heat recovered using Formulas 3 or 4.
Model VHRS (sensible heat recovery)
WINTER
Outdoor Air = 10° FDB, 60% RH
Exhaust Air = 80° FDB, 63° FWB
Supply Airflow 15,600 SCFM
Exhaust Airflow 13,000 SCFM
VHRS Effectiveness (Table 2) = .73
(at 13,000 SCFM and Cabinet Size D)
Wmax / Wmin =15,600 / 13,000 = 1.2
Corrected Effectiveness (Table 3) = .77
Supply Air Leaving Wheel:
T2 = T1-E(Wmin/Wsup) (T1 - T2)
T2 = 10-.77(13,000/15,600) (10-80) 54.9° FDB
Osup = 1.08 Wsup (T2 - T1)
Osup = 1.08(15,600) (54.9 - 10) 756,475
Btuh Exhaust Air Leaving Wheel:
T4 = T3 E(Wmin/Wexh) (T3 - T,)
T4 = 80 .77(13,000/13,000) (80 10)
T4 = 26.1°FDB (see page 5 for frosting considerations)
SUMMER
Outdoor Air = 100° FDB, 77° FWB
Exhaust Air = 780° FDB, 50% RH
Supply Air Leaving Wheel:
T, = 100 - .77(13,000/15,600) (100 - 78) =
85.9- FDB
Qsup = 1.08(15,600) (85.9 - 100)
Qsup = - 237,557 Btuh (cooling) |
