Safety Functions - Carrier AquaSnap 61CG-A Installation, Operation And Maintenance Instructions

7 - OPERATING THE SYSTEM
7.2.3 - Control of peripherals
Peripherals in this context are circulation pumps and 3-way mixing
valves on the source (evaporator) and condenser sides.
Control of circulation pumps via CARRIER heat pump:
The external source and condenser pumps can be controlled by
the heat pump controller. The required prerun and overrun times
are set by CARRIER customer service for each specific application.
The type of control is specified in detail in the system datasheet.
Control of the circulation pumps via external building
management system:
The external BMS fully regulates the targets, controls the system
peripherals (pumps, dampers, valves, etc.) and only requests
active heating or cooling from the heat pump.
In this context, the following functions must be ensured:
■ The source pump must be started approximately 1.0 minute
prior to the heat pump being enabled and must run on for around
1.0 minute after the compressor has stopped (feedback signal).
■ The condenser pump (consumption side) must start up at least
0.5 minutes before the compressor and run on for at least 0.5
minutes.
■ The compressor must not be switched more than 4 times per
hour.
■ The standard minimum operating time must not fall below 6
minutes.
To the extent required, the external BMS receives a feedback
signal about the actual operating state of the compressor by means
of a floating contact. The source and condenser pumps must
always run when the compressor is working to prevent faults and
damage to the heat pump. We recommend linking the prerun and
overrun timers to the enable and feedback signals.
If the source and condenser pumps assume other functions outside
heat pump operation, it makes sense to have the external BMS
control these circulation pumps.
Control of 3-way valves by CARRIER heat pump:
The heat pump control unit can regulate the voltage supply to and
switching of the external 3-way mixing valves (actuators), both on
the source and/or condenser side. In the standard version, the
heat pump issues a 0-10 V target signal to switch the 3-way valves
on the evaporator and/or condenser side. A 4-20 mA signal can
also be provided as an option. The 3-way valves must be installed
in relation to their actuators such that they are fully open towards
the source or consumption side hydraulic system when the 10 V
target signal is applied. When the target signal is 0 V, the valves
should be fully open towards the bypass (short circuit circulation).
A voltage supply is provided to drive the 3-way valves. This can
be either 24 V DC (standard) or 24 V AC.
The voltage supply and type of control are defined in the system
datasheet and may vary from the standard version.
The 3-way valves used must reach an actuating time (duration
from fully open to fully closed state and vice versa) of maximum
60 seconds, 30 seconds is better.
7.2.4 - Buffer tank
The text below describes the heat pump in heating mode as an
example (target at condenser outlet), but it applies vice versa to
cooling mode (target at evaporator outlet).
Note that, depending on the hydraulic system and the temperature
spread, the heat pump target (condenser outlet) generally has to
be higher than the required temperature in the buffer tank. If, for
example, in the heating buffer tank, a temperature of 45°C is
reached, the heat pump target should generally be >45°C
(temperature raising).
Once the buffer tank has been heated to approximately the
required target temperature and the heat pump is off/shutting
down, the heat pump should remain off (via the remote ON/OFF)
until the buffer tank has cooled down again or the agreed
temperature spread across the condenser could be controlled
again at least over the minimum runtime.
If, for example, the target temperature for the condenser outlet is
50°C and the agreed temperature spread between condenser inlet
and condenser outlet is 10 K, the heat pump should only run when
the temperature at the condenser inlet is 40°C or less.
If, for example, the target temperature for the evaporator outlet is
8°C and the agreed temperature spread between evaporator inlet
and evaporator outlet is 10 K, the heat pump should only run when
the temperature at the evaporator inlet is 18°C or more.

7.3 - Safety functions

The following hardware and software safety devices have been
implemented:
■ Motor overload relay for the required source pump
■ Motor overload relay for the required condenser pump
■ High pressure control (high pressure switch or high pressure
limiter)
■ High and low pressure sensors
■ Hot gas temperature monitor
■ Start delay to avoid compressor cycling
■ Frost protection function
■ Source energy pump prerun and overrun time
■ Control cabinet temperature monitoring
■ Flow switch for the evaporator is optionally available and is
recommended by CARRIER for low evaporator inlet
temperatures.
17