The heat gain analysis is generally focused on the offshore and chilled water piping systems as they are the largest potential heat gain contributors. The associated above ground system equipment and piping is a small part of the overall system surface area and is generally located within a building or underground vault. These portions of the system are always insulated as they are exposed to greater ambient temperature variations than the below ground segments.
A number of analyses need to be performed to develop the overall system heat gain picture. During periods of low service loads, the offshore water temperature rise is greater due to the longer residence time in the piping system; but, a smaller heat exchanger approach generally can be achieved at low flows to partially compensate for this temperature rise. Past studies and calculations show that once the ground temperature surrounding the chilled water piping system reaches thermal equilibrium, the heat gain into the system is relatively small. Several factors contribute to the amount of heat gain experienced in the distribution piping system including ground temperature: soil type and moisture content: pipe diameter; burial depth; and the insulation characteristics of the pipe wall system.
For the offshore portion, the DWSC could employ a chemical based water treatment system as a first line of defense to mitigate the potential intrusion and negative effects caused by a marine growth infestation in the intake and outfall areas. Marine growth is known to be pervasive in cooling systems such as the types considered for a DWSC project and needs to be addressed on a long term. preventative basis as well as a remedial basis for established infestations.
The chemical based water treatment system is generally one that is capable of injecting, controlling, monitoring and finally neutralizing, if required, any treatment material residual prior to entering the offshore waters at the outfall. The treatment materials are selected based upon the type and variety of marine life anticipated. The materials are injected at multiple points in the system to provide uniform treatment while the neutralization agent is only injected at a single point. The neutralization agent is injected near the end of the outfall piping to ensure that the neutralization is complete prior to the water being returned to the body of water.
In addition to an offshore treatment system, a mechanical based cleaning system can also be included to allow for the removal of established or dead marine growth formations. Since much of the system internals are inaccessible for manual cleaning, a pressure driven "pigging" type of system has been considered as one means of addressing this issue. This system concept and design is based upon a wealth of experience and a long history of successfal operation in the petrochemical industry and especially, the offshore Gulf Coast applications.
The mechanical cleaning method consists primarily of a pressure driven cleaning device commonly referred to as a "pig", coupled with manual mechanical cleaning of the other remaining and accessible areas of the system. The system employs a cleaning pig which is inserted into the pipe to be cleaned and is propelled through the pipe by water pressure. Depending upon the pig configuration and the amount of buildup intended to be removed, differential pressures of as little as 1 psi will move the pig through the pipe. The system components not cleaned by the pig device must be manually addressed. These portions of the system are the intake structure; the small bore piping systems associated with the pump and heat exchangers; the outfall piping and diffuser. The intake structure should also be designed to be raised to the surface for mechanical cleaning. The small bore piping systems would be flanged and equipped with cleanouts wherever possible to facilitate mechanical cleaning.
The presence of bio-growth and fouling is monitored by measuring system pressure losses and by the visual observation of accessible areas of the system. Based upon these indications, the decision to use water treatment materials and the appropriate application strategy is determined.
The closed loop chilled water side of the system is generally treated upon initial fill with corrosion inhibitors and replenished only to accommodate any system leakage. As such, this side of the DWSC water treatment design is very straight forward.
A typical electrical requirement at peak, for large building air conditioning is approximately 0.85 kWh/ton-hr. This value can vary greatly depending upon the load, ambient conditions and efficiency of the chillers. A typical peak electrical energy requirement for a lake or ocean source cooling system will be in the range of 0.1 - 0.2 kWh/ton-hr. The net effect of a DWSC system is a reduction in energy and instantaneous power consumption attributed to conventional chilled water generation and distribution of 80 to 90%.
Power consumption drops rapidly under part load conditions. This is due to the fact that the bulk of the overall system power consumption is for pumping power. Since the DWSC load capacity is directly proportional to flow (assuming a constant chilled water supply and return temperature differential), the electrical power consumption will drop roughly by the cube of the flow reduction. This, coupled with the plate and frame heat exchangers inherent ability to reduce the approach temperature with a reduction on flow rate, provides for very efficient part load conditions.