COOK Laboratory Exhaust - Manual (Page 14)

5.7 Magnahelic Pressure Gauge (Standard)
"Magnahelic" is not a generic term but is registered by Dwyer Instrument Company. The magnahelic gauge is a
diaphragm operated gauge that has several advantages over a liquid manometer; (1) it need not be leveled to 0
and can be used easily on a ladder or unleveled surface. (2) W hen hooked up to the Pitot tube, it need not be
purged of air bubbles as the liquid manometer may. (3) There is less chance of parallax error reading in the dial
face. (4) It is easily transported without the chance of losing the liquid charge. Unless extreme accuracy is
required, this instrument may replace the manometer for average air conditioning work, and like the manometer,
is available in a variety of ranges. The dial is only 4 inches in diameter and therefore has a limited scale; several
instruments are required to cover the normal ranges encountered in average air conditioning jobs.
5.8 "U" Tube Manometer
Pressure is defined as force per unit area - and the best way to measure air pressure is to balance a column of
liquid of known weight against the air pressure and measure the height of liquid columns so balanced. The units
of measure commonly used are, inches of mercury (in Hg), using mercury as the fluid and inches of water (in
WG.) using water or oil as the fluid. Instruments employing this principle are called manometers. The simplest
form is the basic well known U-tube manometer. This device indicates the difference between two pressures or
between a single pressure and atmosphere, when one side is open to the atmosphere. If a U-tube is filled to the
half way point with water and air pressure exerted on one of the columns, the fluid will be displaced. Thus one
leg of water column will rise while the other falls. The difference in height "h" which is the sum of the readings
above and below the half way point indicates the pressure in inches of water column. The U-tube manometer is
a primary standard because the difference in height between the two columns is always a true indication of the
pressure regardless of variations in the internal diameter of the tubing.
6.0 Coils
A Loren Cook unit may be equipped with chilled water, chilled/hot water energy recovery coils.
6.1 Airflow and Pressure Drops
Air flow for chilled water and direct expansion coils, must not exceed 500 feet per minute to prevent condensate
water from being pulled off the coil and outside the condensate drain pan.
The static pressure drop across each coil will change with the number of rows deep, number of passes, fins per
inch and other design conditions. Consult the written submittal for each unit on this job.
6.2 Hot Water Coils (Summer Conditions)
The temperature rise of the air leaving the coil is also dependent on the airflow across the coil, the gallons of
water flow through the coil, and the entering water temperature into the coil. Consult the submittal for each
specific job for the above information.
BTUH = 500 x GPM x TD
The static pressure drop across each coil will change with the number of rows deep, number of passes, fins per
inch and other design conditions.
6.3 Freeze Protection of Coils
All chilled water, hot water, and steam coils can be damaged during freezing weather. Precautionary measures
must be taken to prevent freezing such as:
1. Draining each coil and related piping, making sure that all low areas also drain.
2. After draining, flush coils with an antifreeze solution such as ethylene glycol. A solution of 50% ethylene
glycol and 50% water will protect from freezing to approximately 35

F below zero at sea level.
3. In case of unit shut down during winter operation, such as unit power failure, night shutdown, and weekend
shutdown, the controls must be installed so the valves will go to the full heat position, all fresh air dampers
and exhaust dampers go to the full closed position, and all return dampers go to the full open position. The
water circulation pumps must keep circulating water through the coils and/or auxiliary heat must be
maintained within the unit cabinet.