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)