4.10 - EXV CONTROL
4.10.1 - General
The EXV is controlled to maintain minimum pinch for insuring the cooler exchange performance through the delta
temperature between the cooler fluid and the saturated suction temperature. EXV opening movement will normally
decrease the cooler pinch and thus increase the cooler exchange performance. But it shall not open too much
provided a drop in discharge superheat , low oil pressure and refrigerant flow.
The EXV control shall deal with two goals : Smaller Cooler Pinch and Discharge Superheat Protection.
Additional protection against excessive EXV opening is applied to ensure the condenser heat transfer (sub cooling).
It may happen when operating in high condensing temperature and indirectly high discharge superheat, exv would
tend to open continuously to get the lower pinch due to the setpoint and unfortunately in certain conditions, this value
has reached the limit, though it is higher than the setpoint. Therefore, EXV position limit shall be set when there is
no more variation in cooler pinch for a variation of EXV position of 4 percent.
EXV position shall also be limited to avoid hunting problem due to low discharge superheat protection. It may occur
when operating in a low condensing temperature, the discharge superheat protection can switch from enable to
disable in few scans and so on.
This control uses a kind of auto adaptive method whose goal is to find out the optimal position of EXV for insuring an
optimal pinch with a correct discharge superheat and sub cooling.
4.10.2 - Overrides
over_exv
Override Description with and priority precedence
.
1
Low ambient startup
2
Low Saturated Suction Temperature
3
Very Low Saturated Suction Temperature
4
Very Low Discharge Superheat
5
Mop control (Closing EXV)
6
Cooler pinch regular control
7
Reclaim injection override
8
High Saturated Suction Temperature (no movement)
9
Low Saturated Suction with High Saturated Condensing Temperature
10,11
Capacity increase
12,13, 14
Capacity decrease
15 Reclaim
pumpdown
session
16,17
Discharge Superheat lower than control point
18, 20
Safeguard Discharge Superheat
19
Exv position limitation active
21,26
Safeguard optimal cooler pinch due high Discharge Superheat or lower exv
position
22
Closing EXV due to good pinch and high motor temperature for economizer unit
23
Exv position limitation active, and go back to previous position
24, 25
EXV limitation is reinitialized
CARRIER SA
USER'S GUIDE FOR
30GX&HX PRO_DIALOG 4 V.2
ECG-UG-02-002
Rev: Original
Page: 45/98
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