Protecting The Unit From Freezing - RHOSS TCCE 135 Gebrauchsanweisung

II.3.3.8 Installation and management of the P/P –DP
circulation pump on the primary circuit
The circulation pump to be installed in the chilled water circuit should be
selected to overcome any pressure drops, at nominal rates of water
flow, both in the exchanger and in the entire water system.
• The differential pressure switch protects the unit from any
interruptions in the flow of water. As it resets automatically, the unit will
restart of its own accord as soon as the water flow returns over the
reset value.
• Once the differential pressure switch has been triggered, the control
panel continues to display the E41 alarm to indicate possible water
system problems (Par. 0).
• The operation of the user pump must be subordinated to the
operation of the unit; the microprocessor controller checks the operation
of the circulating pump according to the following process:
Once switched-ON, the first device to start in the system is the
circulating pump, which has priority over all the other devices. During
the starting phase, the minimum water flow differential pressure switch
fitted on the unit is temporarily excluded, for a preset period, in order to
avoid repeated On/Off caused by bubbling or turbulent flow in the
hydraulic circuit. Once the starting phase is over, final enablement is
given to the machine to start up; 60 seconds from the pump starting,
the fans cut in (during this phase the antifreeze alarm is bypassed);
after a further 60 seconds the compressors start up (allowing for the
safety timer delay). The pump keeps on working all the time the unit is
in operation, and it shuts down only at the stop command.
After switch off, the pump will continue to operate for a pre-set time
period in order to disperse the residual heat in the water heat
exchanger. prior to final shutdown.
Information regarding pump water connections is outlined in enclosed
document 4, which also includes the hook-up diagrams for each
machine version.
If a double stand-by pump (P-P/DP) is present, the second pump is
connected in parallel to the first pump and can be activated if the
working pump malfunctions or breaks-down by acting on a
selector on the control board. (ref. 1, Fig. 11).
II.3.4

PROTECTING THE UNIT FROM FREEZING

II.3.4.1 Unit off - end of season shutdown
IMPORTANT!
If the unit is not used during the winter months, the
water in the system may freeze and therefore
seriously damage the appliance.
To avoid the risk of freezing, the entire contents of the circuit should be
drained before the onset of winter. Use a discharge point below the
water exchanger to assure all the water is drained from the unit. Use
the cocks on the bottom part of the exchangers to make sure the
exchangers are completely empty. If the draindown operation is felt to
be too much trouble, ethylene glycol may be mixed with the water in
suitable proportions in order to protect the system from freezing. The
glycol solution modifies the physical features of the water and,
consequently, the unit performances in terms of efficiency.
Fig. 11
SECTION II: INSTALLATION AND MAINTENANCE
II.3.4.2 While the unit is running
In this case the microprocessor control prevents the exchanger from
freezing. When the set point is reached, the antifreezing alert will be
activated and the unit stopped, whereas the pump will continue working
regularly.
The use of ethylene glycol is only recommended when it is not possible
to drain the water from the circuit during the winter break or if the unit
has to produce chilled water at temperatures below 4°C (the latter case
is not covered herein and depends on the system sizing of the unit).
IMPORTANT!
Mixing the water with glycol modifies the
performance of the unit.
The following table indicates the multiplication factors needed to
determine the variations in performance of the units according to the
required percentage of ethylene glycol.
The multipliers refer to the following conditions: condenser air inlet
temperature 35°C; chilled water temperature 7°C; temperature
differential at the evaporator 5°C (for different operating conditions the
same multipliers can be used since the amount of their change is
negligible).
Min. outdoor air
temperature °C
% of glycol by weight
Freezing temperature °C
fc G
fc ∆pw
fc QF
fc P
Key to table:
fc G = Evaporator glycol water flow rate correction factor
(condenser/evaporator on THCE).
Fc ∆pw = Correction factor for pressure drop at the evaporator
(condenser/evaporator on THCE)
fc QF = Cooling capacity correction factor.
fc P = correction factor of the total absorbed current
II.3.4.3 System water contents
Systems supplied by water chillers usually have limited water
volumes/capacities. In these working conditions (particularly with
reduced thermal loads), the compressor would be forced to start and
stop at too frequent intervals. The microprocessor board protects the
electric motor on the compressor by delaying the next compressor start-
up by 360 seconds once it has stopped. This undermines the efficiency
of the system connected to the unit because the chilled water may be
subjected to excessive temperature variations. We recommend
installing an inertial water storage tank which will, when necessary,
increase the amount of water in the circuit thus drastically limiting the
effect of the water temperature variations. The volume of the storage
tank depends on the type of system, the capacity of the cooling
assembly, the temperature differential of each capacity control step in
the working thermostat. On the basis of the desired inertial effect on the
water temperature, the total quantity of water Q (I), (system + storage),
may be calculated as follows:
Q(l)
P (kW) = Projected cooling capacity.
∆T (°C)
= Working thermostat differential (2 ÷ 6°C).
t (sec.) = Compressor stop time (the delay time is managed by the
microprocessor; to determine the minimum water quantity to
limit the temperature variations when in use, set t=100 sec., +
60 sec. for every minute of delay required).
n (no.) = Number of capacity control steps.
The storage tank should be downstream of the water circuit and
upstream of the chilling plant. This means that the water temperature in
the terminal units is reached as soon as the compressor starts working.
While the compressor is running the water temperature may fall slightly
below the projected value.
39
2 0 -3
10 15 20
-5 -7 -10
1,008 1,028 1,051
1,053 1,105 1,184
0,991 0,987 0,982
0,996 0,995 0,993
P t 1
= ⋅ ⋅ ⋅
860
∆
T n 3600
-6 -10
25 30
-13 -16
1,074 1,100
1,237 1,316
0,978 0,974
0,991 0,989