ERS EMCH-10W/1, Installation And User Manual

Page: 137 / 190

Maximum chilled water flow
The maximum chilled water flow is limited by the permitted pressure drop in the evaporator. It is provided in
the below table. If the system flow is more than the maximum unit flow rate, bypass the evaporator as shown
in the diagram to obtain a lower evaporator flow rate.
Minimum and maximum water flow rates
Item
Water flow rate(m
3
/h)
Model Minimum Maximum
EMCH-07W/1 1.08 1.32
EMCH-10W/1 1.54 1.89
EMCH-12W/1 1.72 2.11
EMCH-14W/3 1.93 2.36
EMCH-16W/3 2.24 2.73
14.6 Design of expansion tank
If a closed expansion tank with its filled volume of air is too small, the system pressure will easily exceed the
maximum allowable pressure and cause water to discharge from the pressure relief valve, thus wasting
water. If the closed tank is too large, when the water temperature drops, the system pressure may decrease
to a level below the minimum allowable value and cause trouble in the air vent. Therefore, accurate sizing of
a closed expansion tank is essential.
For diaphragm expansion tanks, the minimum volume of the water tank, Vt, gal (m
3
), can be calculated by
the following formula, recommended by ASHRAE Handbook 1996, HVAC Systems and Equipment:
V
t
=V
s
v
2
/v
1
-1-3
α
(T
2
-T
1
)
1-p
1
/p
2
T1=lower temperature,
℉
(
℃
)
T2=higher temperature,
℉
(
℃
)
Vs=volume of water in system, gal(m3)
p1=absolute pressure at lower temperature, psia (kPa abs.)
p2=absolute pressure at higher temperature, psia (kPa abs.)
v1, v2 =specific volume of water at lower and higher temperature, respectively, ft3/lb(m3/kg)
α
=linear coefficient of thermal expansion; for steel,
α
=6.5x10-6in./in
·℉
(1.2x10-5per
℃
); For copper,
α
=9.5x10-6in./in
·℉
(1.7x10-5per
℃
)
Evaporator
Recirculation
For minimum chilled water flow rate
Fig. 4-2
For maximum chilled water flow rate
Evaporator
Recirculation
42
V.2