June Copyright Climatec

Transcription

1 June 2013 Copyright Climatec

2 GUIDLINES FOR CENTRAL PLANT OPTIMIZATION The following document provides guidelines which if followed will maximize the energy savings potential for a central plant available through optimization. The attached drawing used is a typical primary variable flow chilled water plant which includes the minimum points required. There are many variations of this design based on size, budgets, and plant requirements but the guidelines will still be applicable. 1 OSA Temperature / Humidity A good quality temperature / humidity must be supplied with at least 3% humidity accuracy. It should be located in a location representative of the building and cooling tower air intakes. It must be located out of direct sunlight at all times throughout the day and out of area that can be heated up due to indirect sunlight, heat, or humidity generating equipment such as a cooling tower or other mechanical equipment. 2 Cooling Tower Selection Select cooling towers with the largest surface area as space and budget will allow. Ensure that low flow nozzles / doers are supplied to allow the best flow turndown possible. These are almost always a special order on cooling tower equipment so early communication with the contractor ordering the cooling tower is necessary. 3 Cooling Tower Isolation / Bypass Valves In a simple 2 chiller 2 tower plant, automatic CT Isolation valves are not required if the cooling towers are selected where the minimum flow for each tower is half the minimum flow of one chiller. If isolation valves are used on open circuit cooling towers make sure the isolation valves are on both the inlet and outlet of the cooling towers. A bypass valve is not typically needed in moderate climates due to the variable speed condenser water pumps. If one is used because of extremely cold conditions it must be installed below the cooling tower sump level.

3 4 Cooling Tower Outlet Temp Individual CT outlet temperature sensors are not required for optimization but are recommended for additional monitoring points and they will allow for fault detection. 5 VFD s Variable Speed Drives for all equipment must be have open protocol communication; preferably BACnet. Most drives provide the necessary points required for optimization through the interface however some may not have atrue kw reading which is required for optimization. Power reading in percent units or deriving power from Amps is not acceptable. 6 Condenser Water Temperatures Condenser supply and return water sensors are required in the common piping between multiple chillers and cooling towers. 7 Chiller Flow If the chillers do not have reliable condenser and evaporator flow readings available from the chiller interface, differential pressure (DP) sensors must be added to determine CHW flow as an analog measurement in gallons per minute (GPM). The following guidelines need to be followed: a. The chiller DP to flow curves need to be obtained from the chiller manufacturers along with the minimum and maximum flow requirements of the condenser and evaporator. Note: A formula as opposed to a graph is preferred for accuracy. The DP sensors should beof the wet-to-wet diaphragm type as opposed to the type with separatehigh and low side transducers with ±0.25% accuracy; recommend Setra Model 230. c. d. The DP sensor range should be selected as low as possible so that the maximum design flow (DP) is approximately in the 75% of the range and the minimum flow (DP) is not below 10% of the range of the instrument. The DP sensors should be installed with a 3-valve manifold for serviceability. Ensure the pressure taps in the pipe are on the chiller side of the isolation valves.

4 8 measurements Chillers should be selected with the lowest NPLV (non-standard part load value) based on the load characteristics the plant will be serving. The chiller must have a network interface which gives all the critical chiller information including water temperatures and refrigerant pressures and temperatures. Onmany chillers, the kw power is an option or not available. If the chiller does not come with a true kw reading then an external power meter will need to be supplied for each chiller. Note: Percent power or calculation from amps is not acceptable. 9 Bypass Valve A modulating bypass valve is required for primary variable pumping systems located on the building side of the chilled water pumps. The bypass valve must be modulating and properly sized for the minimum flow of one chiller. A decoupler is required for primary secondary systems located between the primary and secondary chilled water pump loops. 10 Flow/Energy Meter An Onicon magnetic insertion meter with matchedtemperatures sensors and a BTU (Energy) Totalizer with BACnet interface or similar is highly recommended. An accurate energy reading is critical for the optimization process. The following guidelines must be adhered to: a. The flow and temperature sensors must be mounted on the building side of the bypass valve or, in the case of a primary-secondary system after the secondary chilled water pumps. It is critical that there are enough straight runs of pipe before and afterthe flow meter to allow for accurate flow readings. This is the number one cause for inaccurate energy readings in the plant. c. It is imperative that that the location be coordinated with the mechanical engineer and contractor. Do not trust the drawings alone. Being involved with the shop drawing layout process and onsite location of the flow meter and temperature sensors is a must.

5 11 Plant DP A differential pressure (DP) sensor in the plant is required at a minimum. Optionally, DP sensors can be located near the end of runs or at remote buildings for informational inputs but they are not required if all the chilled water valve positions at the loads are known. If there are areas where valve positions are not known, DP sensors need be located in a representative location near those loads and a minimum DP established for each DP location. 12 Building loads All building loads must be known and understood for proper optimization of the chiller and chilled water pumps. The status of the loads must be available through an Open Protocol network (BACnet preferred). Loadinformation that is highly recommended in order of preference are: a. CHW valve positions. The power (kw) for any variable speed loads that are influenced by CHW temperature. c.` The dehumidification control signal of any humidity sensitive areas.