A chilled water plant’s annual operating cost is a major contributor in a facility budget. Typical chilled water plants consist of multiple chillers, cooling towers, chilled water pumps, condenser water pumps, plate heat exchangers and water treatment equipment.
The chilled water is generated in the central plant and then transported through a piping network to cooling coils (air handlers), or to point of end-use in processes. Facility directors and energy managers are always chasing multiple goals - satisfying all the customers, maintaining a high-level of reliability and minimizing energy spends with varying demand and weather. Therefore, many modern plants employ a good chiller optimization package such as Hudson Technologies’ SMARTenergy OPS® in conjunction with Building Automation Systems (BAS) to optimize the chiller plants.
Chiller Plant Efficiency Driven by Cooling Tower Management
In a water-cooled chiller plant, cooling towers facilitate heat removal. Cooling towers are designed for peak summer conditions - high temperature and humidity. The efficient operation of a chilled water plant is highly dependent on cooling tower management. In practice, four cooling tower management strategies are applied, i.e., constant setpoint, seasonal reset, manual reset, and automatic reset. Application of a strategy is based on the chiller plant configuration and operation philosophy at a particular plant. Automatic reset is the most energy efficient strategy. It uses prevailing ambient conditions and the actual cooling load to continuously make setpoint changes.
The cooling tower energy consumption can have significant impact on the chiller plant performance. The chiller power requirement is dependent on Entering Condenser Water Temperature (ECWT) supplied by the cooling tower. This is explained further in sections below. As the Wet Bulb Temperature (WBT) drops, the chiller can be supplied with a lower ECWT. However, it must be noted the cooling tower approach (ECWT-WBT) increases for the same cooling load, cooling tower flow, and temperature range when WBT drops1. Temperature range is determined by subtracting the ECWT from Leaving Condenser Water Temperature. If the ECWT is set close to WBT without considering the variation in approach, it will be harder to meet the setpoint, which results in higher cooling tower energy consumption. Hence, it is important to manage ECWT in order to get optimum chiller plant performance.
The Impact of Condenser Water Pumps
In a refrigeration cycle, the chiller efficiency is commonly rated in kW/ton. The term is defined as the ratio of the compressor work in kW to the cooling load in tons. The lower the kW/ton – the higher the efficiency is for the chiller.
As the ECWT drops, the pressure in the condenser drops resulting in lower chiller compressor power while the cooling load remains the same. This results in lower kW/ton. Condenser water pumps are an integral piece of the puzzle when optimizing a chiller plant. There are several plant configurations that exist. If all the pumps are fixed-speed pumps and run continuously, their power consumption will not affect the optimum power requirement. However, in the case of Variable Frequency Drive (VFD) pumps, the pump power may vary, contributing to an energy trade-off between chillers, fans, and pumps, which makes it difficult to optimize the system.
Variation in load produces variation in heat rejection and flow for a condenser water pump. For simplicity of analysis for this article, it is assumed that within normal operating ranges, a VFD pump will not have significant energy trade-off with chiller or fans.
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