General Concept:
The general concept of the system is to provide a balancing of chilled water flow to insure the water temperature returning to the chillers is high enough to provide efficient operation of the pumps and chillers.
Atlantic Station was designed with a higher than typical differential temperature (DT) to allow for smaller amounts of water to be pumped, providing a savings for upfront construction, line loss for the miles of chilled water piping, and pumping costs. If the DT is allowed to get too low, there could be a point where there is not enough pump or pipe size to get the amounts of chilled water needed to every tenant. As a bonus to holding the correct DT, the plant operates more efficiently and the tenants save money.
Simply put, we have a signal from the thermostats connected to an input on our control module and if the call for cooling is present, we open the valve to a point where the water leaving the unit is at design conditions. For coils that are clean, sized properly, and have the correct airflow, the discharge air temp goes to the design discharge temp and the tenant space is cooled correctly. An increase in load from the space automatically increases the amount of water flowing through the coil to maintain design.
What you can expect when servicing a unit:
The operation of the system should be no different really than any other chilled water system you are familiar with servicing. If the unit is operating and the thermostat is calling for cooling, then measure the entering and leaving air temperatures of the coil to insure they are at or near design. If the coil is dirty, it will not allow the heat transfer as it should and the water will start coming out too cold. This will be corrected by adjusting the valve slightly closed. Over time, the degradation in unit performance will result in the space not cooling properly and routine maintenance focused on coil cleaning should be performed.
On a call for cooling, the valve will go to the last known position that satisfied the load. The thought being that if the unit cycled off 10 or 12 minutes ago (typical 6 turns per hour) then the load would not have changed much and the last valve position is a good place to start. From there, the valve will modulate around open or closed slowly (minutes) to maintain the temperature coming out of the coil as described.
If the unit has been powered cycled, then there is no last valve position to start at; therefore, it starts at a minimum position to insure the coil is flooded and waits a variable delay time for the heat of the coil to start coming out. Then the valve will open to a position that puts the water temperature either near setpoint or sufficiently moving toward setpoint. Fine adjustments from there will continue as needed to get the valve to the exact position needed. When the thermostat no longer has a call for cooling, the valve will close. The initial opening after a power cycle can take several (8 to 10) minutes for the coil performance to be confirmed.
Offsite Assistance
If you need the valve to be full open to measure coil DP or other tests, or if you want AFS assistance during your service call. We can usually connect to the system remotely and override the system to allow you to verify proper operation for the tenant. This service should be scheduled in advance so we will be prepared to help you. The service can be paid for via credit card directly or billed to your firm and you add a line item on your work ticket.