CEMP-E
TI 811-12
18 August 1998
installations in the northern hemisphere, mounting of sun shields on the southerly exposure of a building
will be avoided.
b. Current sensing relays may be used for motor status feedback, where constant motor running
horsepower allows the relay to be set for approximately fifty percent of full load. Current transducers will
be used for variable speed or variable load motor status feedback, and may be applied to other large
loads where precise energy consumption measurements are required.
c.
Switches for UMCS use will have the following characteristics and be applied as follows:
(1) Differential pressure switches may be used for monitoring and alarming air filter loading on
constant volume air systems. Differential pressure sensors will be considered for variable volume air
systems.
(2) Pressure switches must have adjustable settings, and be selected to have the switch setting
in the middle half of the device's range.
(3) Temperature switches meeting the accuracy requirements may be used in lieu of
temperature sensors where an analog readout is not required.
d. Selection of flow sensors will include consideration of accuracy, rangeability, and physical
installation requirements. The designer will perform the appropriate calculations as described in TM 5-
815-3, HVAC Control Systems. Flow sensors will be applied as follows:
(1) The required accuracy of a measurement will be determined based on the intended use of
the flow information by the UMCS. Flow measurement of compressible fluids such as steam will be
compensated by temperature and pressure measurements when high accuracy is required.
(2) Rangeability is the ratio of the maximum to minimum flows over which the flow sensor
maintains the specific accuracy. The required rangeability of a flow sensor will be determined based on
the anticipated variations in process flow conditions, such as seasonal variations in steam and chilled
water flow.
(3) The physical installation of liquid flow sensors requires minimum straight runs of pipe both
upstream and downstream of the sensor, which vary depending on the specific sensor type and whether
or not straightening vanes are installed. The designer will consider these requirements in the selection
and location of flow sensors. In general, the longest straight run of pipe available will be selected, with
consideration of maintenance access and clearance requirements for hot-tap tools (where applicable).
(4) There may be a significant pressure drop across head type flow sensors (orifices, flow
nozzles, venturi tubes) and volumetric displacement type flow sensors. The designer will consider the
impact of flow sensor pressure drop on system operation. Detailed application and installation
requirements for the use of head-type primary flow measuring devices, and the secondary measuring
elements (differential pressure transmitter), are described in the ASME publication "Fluid Meters, Their
Theory and Application".
(5) Turbine flow sensors provide excellent accuracy and rangeability and will be considered for
clean chilled water and hot water flow measurements. However, turbine meter heads are susceptible to
damage from suspended solids in dirty liquids or from slugs of condensate in steam systems. Insertion
turbine flow sensors are installed using hot-tap methods without shutting down the process system.
(6) Vortex shedding flow meters provide excellent accuracy and rangeability and will be
considered for steam flow measurements. Insertion vortex shedding flow sensors are installed using hot-
tap methods without shutting down the process system.
12-9
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