246 lines
6.3 KiB
Markdown
246 lines
6.3 KiB
Markdown
---
|
|
id: 2026-06-04T10:05:03-0400
|
|
title: 2026-06-04 10:05:03
|
|
tags: []
|
|
daily: "[[2026-06-04]]"
|
|
---
|
|
# 2026-06-04 10:05:03
|
|
|
|
## Mechanical Rules of Thumb
|
|
|
|
### Cooling towers
|
|
|
|
Avoid single cell tower
|
|
|
|
Avoid tower without a bleed-off
|
|
|
|
Avoid tower without heat trace and insulation on the make-up water
|
|
|
|
Avoid line-to-line bypass (in lieu of dumping into the sump)
|
|
|
|
### Chillers/chilled water
|
|
|
|
|
|
Avoid one chiller system (exception: dual circuit air-cooled)
|
|
|
|
Avoid series chillers without bypasses
|
|
|
|
Avoid a coil with more than 14 fins/inch
|
|
|
|
### Equipment/misc.
|
|
|
|
Avoid simplex air compressor
|
|
|
|
Avoid simplex sump pump
|
|
|
|
Avoid dx built-up systems (vav)
|
|
|
|
Avoid coil face velocity above 550 fpm (could send condensate down ductwork)
|
|
|
|
Avoid horizontal cold water lines or horizontal roof drain lines above ceilings without insulating (dripping/sweating)
|
|
|
|
Isolation valves on both sides of all equipment - good practice
|
|
|
|
Think about future service: don't locate where dangerous to service
|
|
|
|
Avoid discharging roof drains onto a driveway or where pedestrian traffic
|
|
|
|
Avoid floor mounted water closets in commercial buildings - cleaning issue
|
|
|
|
Minimum 4" housekeeping pads underneath all floor mounted equipment
|
|
|
|
Ashrae recommends 400 fpm face velocity maximum at intake louvers
|
|
|
|
Avoid standard filters at more than 500 fpm (see mfgr recommendations)
|
|
|
|
Trap primers on all floor drains
|
|
|
|
### Ashrae 62 (ventilation)
|
|
|
|
* 20 cfm/person oa in office buildings.
|
|
exceptions: large assembly areas
|
|
|
|
* 15 cfm/person oa in schools.
|
|
|
|
note: this ventilation might exceed the local codes,
|
|
but ashrae recommendations should be met.
|
|
|
|
### Smacna pressure class
|
|
|
|
* 1" wg - low pressure
|
|
* 3" wg - medium pressure
|
|
* Under 3" wg - smacna does not recommend duct testing
|
|
|
|
If higher static pressures are specified,
|
|
try to get lower pressure classes approved.
|
|
The savings in duct costs are significant.
|
|
|
|
### Ductwork sizing
|
|
|
|
| Case | Max Friction Per 100ft |
|
|
| ------------ | ---------------------- |
|
|
| Low pressure | 0.10" |
|
|
| Exhaust duct | 0.15" |
|
|
|
|
|
|
| Case | Velocity FPM |
|
|
| --------------------- | ------------------------------- |
|
|
| Medium pressure riser | Max 3000 |
|
|
| Medium pressure | Max 2500 |
|
|
| Low pressure | Max 1800-2000 |
|
|
| Return air shaft | Max 1000 |
|
|
| Outside air shaft | Max 1000 |
|
|
| Grease duct | Target 1800, Min 1500, Max 2200 |
|
|
|
|
Due to excessive noise,
|
|
Avoid fan-powered or VAV boxes larger than 2000 CFM.
|
|
|
|
Install a balance damper at each low-pressure runout to diffusers.
|
|
|
|
No more than 90° turn with flex duct, straight duct run into boxes.
|
|
|
|
Where space permits,
|
|
consider converting rectangular ductwork to round ductwork.
|
|
|
|
Energy code requires all duct to be sealed.
|
|
|
|
### Pumps
|
|
|
|
Try to locate condenser water pumps as low as possible
|
|
as compared to the cooling tower basin - NPSH - gravity flow.
|
|
|
|
Select pumps for "non-overloading".
|
|
|
|
Select impeller no larger than 90% of maximum impeller size.
|
|
|
|
### Air units
|
|
|
|
| Tons | CFM/Ton |
|
|
| ------------- | ------- |
|
|
| under 20 | ~400 |
|
|
| 25 and larger | ~350 |
|
|
|
|
|
|
Avoid VFD bypass on VAV fan systems
|
|
because it can blow apart duct in manual mode.
|
|
Consider a spare VFD, or dump damper/blow-out doors.
|
|
|
|
Avoid larger economizer systems without powered exhaust.
|
|
|
|
### Useful formulas (standard conditions)
|
|
|
|
Quantity (CFM) = Velocity (FPM) * Area (SF)
|
|
|
|
Sensible load (BTUH) = 1.08 * CFM * (Delta-T)
|
|
|
|
Latent load (BTUH) = 0.68 * CFM * (Delta-GR)
|
|
|
|
Total load (BTUH) = 4.45 * CFM * (Delta-H)
|
|
|
|
* Formula for water:
|
|
* Load (BTUH) = 1.08 * CFM * (Delta-T)
|
|
|
|
$$
|
|
\rm{GPM} = \frac{\text{Load (BTUH)}}{500 \times \text{Delta-T}}
|
|
$$
|
|
|
|
* A/C equipment efficiencies:
|
|
|
|
$$
|
|
|
|
$$
|
|
|
|
### Conversion factors
|
|
|
|
* 1 PSI = 2.307 ft of water
|
|
* 1 ton A/C = 12,000 BTUH
|
|
* 1 hp = 2545.1 BTUH = 745.7 watts
|
|
* 1 kW = 341 5 BTUH
|
|
* 1 cubic foot = 7.48 gallons
|
|
* 1 gal. of water = 8.345 lbs
|
|
|
|
### pipe sizing
|
|
|
|
* 8 to 10 FPS max. Velocity
|
|
* 6 FPS max. Velocity (public areas - noise)
|
|
* 10 to 15 FPS velocity in central plant headers or on roof
|
|
* 3" ano smaller: max head loss 6 ft/100 ft
|
|
* Schedule 40 blk. Stl. Pipe sizing: (* std wt)
|
|
|
|
### Hvac piping
|
|
|
|
* Chilled water: 2 GPM/ton (12°F Delta-t)
|
|
* Condenser water: 3 GPM/ton
|
|
|
|
#### Thermal Expansion
|
|
|
|
(steel pipe): 1" expansion/100 ft of pipe/100°F rise
|
|
|
|
### A/C condensate/waste sizing
|
|
|
|
| Tons | Pipe Size |
|
|
| -------- | --------- |
|
|
| Up to 20 | 3/4" |
|
|
| 21--40 | 1" |
|
|
| 41--90 | 1-1/4" |
|
|
| 91--125 | 1-1/2" |
|
|
| 126--250 | 2" |
|
|
|
|
### Air device sizing
|
|
|
|
| CFM | Neck Size |
|
|
| --------- | --------- |
|
|
| Up to 150 | 6" |
|
|
| 151--250 | 8" |
|
|
| 251--450 | 10" |
|
|
| 451--650 | 12" |
|
|
| 651--800 | 14" |
|
|
|
|
### FPB/VAV sizing
|
|
|
|
| Inlet Size | Max CFM |
|
|
| ---------- | ------- |
|
|
| 6" | 450 |
|
|
| 8" | 700 |
|
|
| 10" | 1200 |
|
|
| 12" | 1700 |
|
|
| 14" | 2300 |
|
|
|
|
Note: runout distance is very important.
|
|
The inlet size shown is not necessarily the correct runout size to the box.
|
|
(Medium pressure runs 0.3 ft/100 ft max)
|
|
|
|
### Cooling loads
|
|
|
|
| Building Type | SF/Ton |
|
|
| ------------------ | -------- |
|
|
| Office buildings | 300--325 |
|
|
| Tempered warehouse | 500 |
|
|
| Call center | 275--300 |
|
|
|
|
* Typical air distribution for office bldgs:
|
|
* north 1.5 CFM/SF
|
|
(assumes 12 ft zone east 2.0 CFM/SF and 25°F Delta-t)
|
|
* south 2.5 CFM/SF
|
|
* west 2.5 CFM/SF
|
|
* interior 0.8 CFM/SF
|
|
* total should avg 0.8 to 1.0 CFM/SF gross
|
|
|
|
* Smoke evacuation: 1 CFM/SF
|
|
|
|
* Stairwell pressurization: 2000 CFM/floor
|
|
|
|
### Plumbing Fixture Runout Minimum Size Schedule
|
|
|
|
| Fixture | Waste | Vent | Cold | Hot |
|
|
| ---------------------- | ------ | ------ | ---- | ---- |
|
|
| Water Closets | 4" | 2" | 1" | - |
|
|
| Lavatories | 1-1/2" | 1-1/2" | 1/2" | 1/2" |
|
|
| Urinals | 2" | 1-1/2" | 3/4" | - |
|
|
| Sinks | 2" | 1-1/2" | 1/2" | 1/2" |
|
|
| Service Sinks | 3" | 2" | 3/4" | 3/4" |
|
|
| Hose Bibbs | - | - | 3/4" | - |
|
|
| Box Hydrants | - | - | 3/4" | - |
|
|
| Electric Water Coolers | 2" | 1-1/2" | 1/2" | - |
|
|
| Showers | 2" | 2" | 1/2" | 1/2" |
|